gh-96052: codeop: fix handling compiler warnings in incomplete input (GH-96132)

Previously codeop.compile_command() emitted compiler warnings (SyntaxWarning or
DeprecationWarning) and raised a SyntaxError for incomplete input containing
a potentially incorrect code. Now it always returns None for incomplete input
without emitting any warnings.
(cherry picked from commit 426d72e7ddb0af5cf851914ac75127186dd1ff04)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>