maven_deploy: allow empty SNAPSHOT_REPOSITORY or RELEASE_REPOSITORY
maven_deploy: allow empty SNAPSHOT_REPOSITORY or RELEASE_REPOSITORY

Allow empty strings, so it becomes possible to select between the old
S3-based JAR storage (used later by Kurento's Archiva server, which we
want to remove) or the default of kurento-parent-pom which is the newer
GitHub registry.

Kurento/adm-scripts,Kurento/adm-scripts