When computing a vector to be used as a key for an index lookup, do not

When computing a vector to be used as a key for an index lookup, do not
check for NULL values and abort until after all key values have been
computed, in case one of the later key values involves some initialization
that is needed by a LEFT JOIN.  Fix for the problem identified by
[forum:/forumpost/ab95010d410a0a55|Forum post ab95010d410a0a55].

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