Fix PHP 8.1 "explode(null)" exception which blocks rendering the Multimeter page master
Summary: Passing null as an input string to explode() is deprecated in PHP 8. Thus first check via `phutil_nonempty_string()` that the input string is not null. Note: this may highlight other absurd input values that might be worth correcting instead of just ignoring. If phutil_nonempty_string() throws an exception in your instance, report it to Phorge to evaluate and fix that specific corner case. Closes T15361 Test Plan: Applied this change; afterwards `/multimeter/` correctly rendered in web browser. Reviewers: O1 Blessed Committers, valerio.bozzolan Reviewed By: O1 Blessed Committers, valerio.bozzolan Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno Maniphest Tasks: T15361 Differential Revision: https://we.phorge.it/D25254