Change erroneous loose comparison to strict comparison in tab panels

Summary:
Fix a loose comparison causing a bug when comparing the selected tab (0) with a tab which has an alphanumeric ID which doesn't begin with an integer.

E.g., (0 == 'kq3p37awi2n5') is true in PHP 7.4 and below, this can sometimes cause multiple tabs to be displayed when a tab panel is first loaded onto a page:

{F278735}

Test Plan: Create a tab panel with at least 3 tabs, add a couple more and ensure that multiple tabs aren't loaded.

Reviewers: O1 Blessed Committers, valerio.bozzolan, Cigaryno, avivey

Reviewed By: O1 Blessed Committers, valerio.bozzolan, Cigaryno, avivey

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15146

Differential Revision: https://we.phorge.it/D25067