Null check signature array before comparison

SigningDetails.UNKNOWN is defined with a null array, so if any
package fails to scan signatures, this code would crash with an
NPE.

Instead of failing, just treat it as an empty signature array.

It's unclear what caused this to pop up now, as this code hasn't
changed in a long while.

Bug: 223469467

Test: presubmit

Change-Id: Icdcfaaa3586898873b719c74dd5662b03e3fabae