Check array key exists before access in Xlsx.php

[jarrettj]

When loading an xlsx file which has images that use a URL, there's a PHP Notice that is shown.

Added a check for the existence of the array key for accessing.

This is:

- [ x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Accessing invalid keys in arrays should be checked and skipped over.