Avoid attempting to refer to an unknown index in views alter hook

[adam-vessey]

When running in a bare site (without any content... like, in a unit
testing rig), this ends up trying to refer to a non-existent offset.

... add an isset() test to avoid doing so.

GitHub Issue: This one.

• Other Relevant Links (Google Groups discussion, related pull requests,
  Release pull requests, etc.)

N/A

What does this Pull Request do?

Tests of the existence of views data being copied before attempting to reference it, as it may not exist.

What's new?

Just added an isset() check before blindly attempting to use views data.

• Does this change add any new dependencies? No.
• Does this change require any other modifications to be made to the repository
  (i.e. Regeneration activity, etc.)? No.
• Could this change impact execution of existing code? Shouldn't.

How should this be tested?

Good question... kind of surprised this doesn't pop up in this module's tests...

Documentation Status

• Does this change existing behaviour that's currently documented? Shouldn't.
• Does this change require new pages or sections of documentation? No.
• Who does this need to be documented for? Unknown
• Associated documentation pull request(s): ___ or documentation issue ___

Additional Notes:

Any additional information that you think would be helpful when reviewing this
PR.

Interested parties

Tag (@ mention) interested parties or, if unsure, @Islandora/8-x-committers

[whikloj]

I can't see how this causes any problems, I'll try to test it this week.