R1:a88dc2afc2a9

Added a setup check for empty REMOTE_ADDR

Summary: Fixes T8850. Previously, if a user's preamble script mangled `$_SERVER['REMOTE_ADDR']` or somehow set it to `null`, the user would get errors when performing certain actions. Now those errors shouldn't occur, and instead the user will be warned that there is a setup issue related to their preamble script.

Test Plan: Create a preamble script that contains `$_SERVER['REMOTE_ADDR'] = null;` then navigate to /config/issue/. There should be a warning there…