Merge branch '361-build-system-bug-in-loading-used-overlays' into 'master'

Resolve "build-system: bug in loading used overlays"

Closes #361

See merge request Pmodules/src!369

(cherry picked from commit afdaa632a0)

e8f38df1 build-system: bugfix if 'use_overlays' is empty

Co-authored-by: gsell <achim.gsell@psi.ch>

Pmodules/libpbuild.bash

@@ -862,6 +862,7 @@ _build_module() {
 	}
 
 	load_overlays(){
+		[[ -n ${ModuleConfig['use_overlays']} ]] || return 0
 		eval "$( "${modulecmd}" bash use ${ModuleConfig['use_overlays']} )"
 	}