Fix undefined index error

[bondjimbond]

Fixes #38

What does this Pull Request do?

Adds a check to make sure the index we require is actually present in the response, so that we avoid generating a thousand Undefined Index errors with every search.

What's new?

Just a simple IF statement.

How should this be tested?

• Define some facet replacements.
• Run an empty search in a fairly full repository and view the logs. Note Undefined Index errors.
• Check out this PR.
• Rerun the search.
• View logs; should be no more Undefined Index errors.

[bondjimbond]

Can't see any reason not to merge.