CSSMin: Don't match empty string as remappable url

CSSMin: Don't match empty string as remappable url

The empty string being matched causes an undefined array index
notice in production, seen from various random gadgets, but spiked
after a change in MonoBook from last week that introduced a
broken background-image rule with empty string as url.

In browsers, that is actually interpreted as valid and "expands"
to the current url and re-fetches as Accept:image/*, silly, but
still broken. The broken icon was fixed in MonoBook, but we still
need to avoid trying to remap empty string as url.

Two changes:

1. Fix regex used by remap() to not match empty string.
   This was already fixed for the 'url()' case without the
   optional quotes, but with quotes, it was being matched as
   non-empty. This is now fixed by using '+' instead of '*'.
   Added tests to confirm they produce output, and PHPUnit
   is configured to also assert no Notices are emitted (which
   it converts to fatal exceptions).

2. Fix processUrlMatch() as sanity check to throw if the key
   is missing.

Bug: T191237
Change-Id: I0ada337b0b4ab73c80236367ff79c31bcd13aa7d