Stop coalescing NULL to empty string in key provider functions

[jeltz]

Since we can pass NULL values in the functions which takes JSON it makes no sense to not do so in the functions which take text. By doing this we only risk not excercising some code paths.

[AndersAstrand]

Does anyone involved when these were added remember why it was done? I'm all for removing them unless we have really good reasons to have them, as they might hide bugs

[dutow]

Can we also add some testcases that uses nulls with this?

If I remember correctly the original json parser accepted empty strings, but reported an error on nulls, which wasn't ideal for optional parameters such as a CA file.