[INTERNAL] Fix to LICENSE.txt available as artifact (SAP internal)

Avoid the execution on the projects which do not provide a LICENSE.txt
file, e.g. the UI libraries and TestSuite which reuse the reactor as
parent POM but don't need to attach that file.

Change-Id: I0fd93d0bb1bc7e4dcf797ecaa8d1961bc2eaa929
Merge "[INTERNAL] Fix to LICENSE.txt available as artifact (SAP internal)"