remove last maven: quirks and make it always a string

It turns out that the maven: field was originally declared as a TYPE_STRING,
given that it was not given a different type in metadata.py's 	flagtypes.
The code was confused because it was given a default value of `False` rather
than `None` as the rest of the TYPE_STRING fields have.

This construct in build.py means maven: should always be a string:

        if '@' in build.maven:
            maven_dir = os.path.join(root_dir, build.maven.split('@', 1)[1])
        else:
            maven_dir = root_dir

.gitlab-ci.yml

@@ -58,6 +58,7 @@ metadata_v0:
     - ../tests/dump_internal_metadata_format.py
     - sed -i
           -e "/buildozer/d"
+          -e 's,maven\(\W\) false,maven\1 null,'
           metadata/dump_*/*.yaml
     - diff -uw metadata/dump_*