Prevent undefined index warnings

In some languages the \w+ does not match the characters used
when translating UTC and the regular expression attempting to
match the timezone fails. Testing in prod wikis where this fails
such as ne.wikipedia.org shows it still works, it just generates
a more generic regular expression.

Since the overall process still works acceptably on the wikis outputting
warnings this patch just adds a guard to prevent the warning and does
not attempt to fix the underlying issue.

Bug: T76558
Change-Id: If8e1ddd2d642b042cc24c51d5ba5aa8b34bc9552
Merge "Prevent undefined index warnings"