mpbb: skip install-deps if port has no deps

If a port does not have any dependencies, the port command fails:
$ port install depof:bzip2
Error: No ports matched the given expression

Check for the list of dependencies first and skip this step if it is empty.

git-svn-id: https://svn.macports.org/repository/macports/contrib/mp-buildbot@146894 d073be05-634f-4543-b044-5fe20cf6d1d6

mpbb-install-dependencies

@@ -15,6 +15,11 @@ install-dependencies() {
         return 1
     fi
 
+    # skip if the port has no dependencies
+    if [ -z "$(${option_prefix}/bin/port echo depof:${option_port})" ]; then
+        return 0
+    fi
+
     if ! "${option_prefix}/bin/port" -d install --unrequested depof:"${option_port}"; then
 
         echo "Build failed, cleaning up..."