Determine which String methods that can skip nullchecks

[mpirvu]

PR #13465 introduced a change where all String methods are no longer eligible for skipping nullchecks for Java17 and up. This is was done to address a problem with String.split which does require a nullcheck in its reference implementation.
We should examine String methods and decide on a case by case basis which methods can skip nullchecks and which can't.

[vij-singh]

@mpirvu How has this been progressing? Given we are trying to get Java 17 released, is this something we want to do for 0,29.1 or 0,30.0?

[mpirvu]

I am going to work on this in the next couple of days.
I think we should postpone this for 0.30.0 because there is little impact on performance and we don't want to jeopardize 0.29.1 so late in the game.

[0xdaryl]

@mpirvu : do you have something to contribute to 0.30 for this? If not, let's push it forward.

[mpirvu]

I intended to work on this, but I was side-tracked to deal with other issues. Let's push it to the next release.