Title: Add warning if newFromText is given non-string/non-null value

Title: Add warning if newFromText is given non-string/non-null value

Specify null as allowed value to make this method more easy to use.
The method is often used like that already and this makes it official.
Previously we were on a months-long trajectory to try and remove all
instances of passing null, but it was a never-ending quest (with new
ones introduced all the time). And in all cases the added handling
in the caller is the same as the handling for when this method
returns null. Requiring the caller to bounce null makes code more
complex, duplicates logic and serves no reason. Let this method be
a non-throwing validation method that returns null if given null.

Bug: T76305
Change-Id: I47517021471ac6fd6daca6deee039c8047224a33