Fix project.properties dependency issue
Correctly place scrlib references also in the case there is no project.properties file present in the original repository.
This commit fixes a problem, which was introduced with 021df3c42420de542bf11f22948831023b47ebf3: If no project.properties file exists in the original project's source repo, place_srclib will not place a dependency --- even if there should be one:
if not os.path.isfile(proppath): return
Example app: fdroiddata/metadata/org.berlin_vegan.bvapp.txt