Adds test and lets us test against empty strings

[henriksen]

Summary of the changes (Less than 80 chars)

• When using JsonPatch.Test, this removes check for current value being null or empty.

Addresses #28019

This makes it compliant with RFC6902 4.6 saying

Here, "equal" means that the value at the target location and the
   value conveyed by "value" are of the same JSON type, and that they
   are considered equal by the following rules for that type:

   o  strings: are considered equal if they contain the same number of
      Unicode characters and their code points are byte-by-byte equal.

Two empty strings should be equal.

Might also want to remove the null check on current value completely, seeing as RFC6902 specifies that literals (false, true, and null): are considered equal if they are the same. Testing a null value against a null value would now fail. As I read the spec, they should be considered equal.

[dnfadmin]

All CLA requirements met.

[mkArtakMSFT]

Thanks for your PR, @henriksen
@pranavkm can you please review this? Thanks!

[henriksen]

Let me know if removing the null check should be a separate PR, or if I should just include it here.

[pranavkm]

@henriksen could you send a separate PR for the null change? Thanks!