Before this commit, questionable optimization existed which
unwrapped string interpolating call with single argument to this argument.
However, this led to source element loss and the necessity of sub-hacks.
In this commit we dropped this optimization (anyway user can remove
this single-expression string template in code if needed) to keep
source elements intact.
backends by removing IMPLICIT_NOTNULL casts from IrStringConcatenation
arguments.
Also fixed an issue where IrStringConcatenation can be lowered into
a null String instead of a literal "null" String if the lone argument
was a platform type String or String with enhanced nullability and the
value was null (e.g., "${FromJava.nullPlatformString()}").