Editorial: MakeMatchIndicesIndexPairArray: avoid creating a property on undefined (#2714)

This is currently a _potential_ spec bug, not an actual one. This AO is not called such that this would be an error, but there are no assertions to prevent it from being one.

Author: ljharb

spec.html

@@ -36322,6 +36322,7 @@ <h1>
                 1. Let _matchIndexPair_ be *undefined*.
               1. Perform ! CreateDataPropertyOrThrow(_A_, ! ToString(𝔽(_i_)), _matchIndexPair_).
               1. If _i_ &gt; 0 and _groupNames_[_i_ - 1] is not *undefined*, then
+                1. Assert: _groups_ is not *undefined*.
                 1. Perform ! CreateDataPropertyOrThrow(_groups_, _groupNames_[_i_ - 1], _matchIndexPair_).
             1. Return _A_.
           </emu-alg>