Explicitly cast potentiall nulls to array for array_merge()

[smgallo]

Description

If resource codes are specified for either inclusion or exclusion the mapping between resource codes and database ids is loaded and the resource codes are verified against this mapping. If only inclusion or exclusion resources were specified, a null would be passed to array_merge() causing an error. This PR casts the value to an array to mitigate this.

Motivation and Context

Fixes error when running ETL on XSEDE

Warning: array_merge(): Argument #1 is not an array in /data/www/xdmod/share/classes/ETL/EtlOverseer.php on line 185

Tests performed

Manual testing on XSEDE development instance.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project as found in the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.