how can i make the loop restart when i press enter?

Im learning C#, and i want to repeat part of my code, if person types not a digit. SO, if person types a letter, everything seems fine, and the loop repeats. But, if the person just click enter, program breaks. First loop works correct, if you click enter, it starts again, but the seond, idk why, but it recongises enter(nothing) as true. Because of this, my formula cant multiply nothing, and i get an error. So, if the bool think that nothing IS a digit, how can i fix this? ( the problem is in first case ).

using System;
using System.Text;

class Geeks
{

    // Main Method
    public static void Main()
    {
        //buy
        float kg = 1000;
        string bananas;
        float BananaKgPrice = 1.69f;
        float bananaWeight = 200;
        float WatermelonKgPrice = 3.29f;
        double price;
        string BuyChoice;

        Console.WriteLine("Welcome to our shop! We sell bananas and watermelons.");
        Console.WriteLine($"Kilo of bananas costs {BananaKgPrice} euros, kilo watermelons {WatermelonKgPrice} euros");
        bool loopBreakBuyChoice = true;
        while (loopBreakBuyChoice)
        {
            Console.WriteLine("What do you want to buy? 1 - banans, 2 - watermelons");
            BuyChoice = Console.ReadLine();
            switch (BuyChoice)
            {
                case "1":
                    loopBreakBuyChoice = false;
                    Console.WriteLine($"Price of bananas for kilo.: {BananaKgPrice} euro");
                    bool loopBreakBananaDigits = true;
                    while (loopBreakBananaDigits)
                    {
                        Console.WriteLine("How much bananas do you want to buy?");
                        bananas = Console.ReadLine();//press enter
                        bool DigitsBanana = bananas.All(char.IsDigit);
                        if (DigitsBanana != true)
                        {
                            loopBreakBananaDigits = true;
                            Console.WriteLine("Please insert AMOUNT of bananas.");
                        }

                        if (DigitsBanana == true)
                        {
                            loopBreakBananaDigits = false;
                            loopBreakBuyChoice = false;
                            price = Math.Round(Convert.ToDouble(bananas) * Convert.ToDouble(bananaWeight) / Convert.ToDouble(kg) * Convert.ToDouble(BananaKgPrice), 2);
                        }
                    }
               break;
            }

        }
    }
}

I alredy tried to fix my problem here, but i did some problems in my post, so it was closed. Re-trying to make this post, and fix my problem.

If you press enter without entering anything Console.ReadLine will return an empty string. Enumerable.All is implemented that it returns ...

> true if every element of the source sequence passes the test in the
> specified predicate, or if the sequence is empty; otherwise, false.

So i think this fixes the issue:

bool DigitsBanana = !string.IsNullOrEmpty(bananas) && bananas.All(char.IsDigit);

Another approach would be to use int.TryParse because you are actually trying to parse the banana-count:

int bananaCount = 0;
while (true)
{
    Console.WriteLine("How much bananas do you want to buy?");
    if (int.TryParse(Console.ReadLine(), out int count))
    {
		bananaCount = count;
		break;
    }  

    Console.WriteLine("Please insert AMOUNT of bananas.");                 
}

price = bananaCount * Convert.ToDouble(bananaWeight) / Convert.ToDouble(kg) * Convert.ToDouble(BananaKgPrice), 2);

When programming you need to learn to abstract concepts. In your example you need to input a line, check that it is an integer and maybe check that it's within certain limits. So you implement a function to do that, call it with the right parameters and you've abstracted away the trouble and the main code looks cleaner.
You can implement a method like this:

public int GetIntegerFromKeyboard(string prompt, int minimum = 0, int maximum = Int32.MaxValue)
{
	while (true)
	{
	    Console.WriteLine(prompt);
		string s = Console.ReadLine();
		bool isInteger = Int32.TryParse(s, out int i);
		if (isInteger)
		{
		    if (i>=minimum && i<=maximum)
			{
			    return i;
			}
		}
	}
}

And you can call it like this:

int buyChoice = GetIntegerFromKeyboard("What do you want to buy? 1 - banans, 2 - watermelons", 1, 2);
...
int amount = GetIntegerFromKeyboard("How many kilograms of bananas do you want to buy?", 1);
...

After these calls you know that the return value is a valid integer.

Some other things:
Following conventions local variables start with lowercase thus it's

float bananaKgPrice = 1.69f;
float watermelonKgPrice = 3.29f;
string buyChoice;

And comparing a boolean to true or false looks amateurish. So prefer

if (!digitsBanana)

over

if (DigitsBanana != true)