Added nullable helper for IfNullOrWhiteSpace

[patrickdemooij9]

I noticed while working on a project that IsNullOrWhiteSpace always demanded that you null check the result even if you are 100% sure that result cannot be NULL. This is because there were no helper classes on it that could help with this. If defaultValue is not null, then we can be sure that the return value is not null. So I've added the helpers to the function.

Take the following code:

public void Test()
        {
            string? testNullString = null;
            string testString = "Hello";

            string test = testNullString.IfNullOrWhiteSpace("Hello")!;
            string? test2 = testNullString.IfNullOrWhiteSpace(null);
            string test3 = testString.IfNullOrWhiteSpace(null)!;
        }

This is how you would have to write it now. Even though we know test cannot be null, we still have to add the ! to the end of the statement to force it to accept a normal string.

However with the changes of the PR, you will be able to write it like this:

public void Test()
        {
            string? testNullString = null;
            string testString = "Hello";

            string test = testNullString.IfNullOrWhiteSpace("Hello");
            string? test2 = testNullString.IfNullOrWhiteSpace(null);
            string test3 = testString.IfNullOrWhiteSpace(null)!;
            string? test4 = testNullString.IfNullOrWhiteSpace(null);
        }

So now we don't have to add the ! after test as we already know that it will never return null.

[github-actions]

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[mikecp]

Thanks @patrickdemooij9 for this nice little fix!
Cheers!