repository: fixup protocol must ignore novalue / unknown

Use the get_source_code_repository helper instead of looping over the
claims to discard the source code repositories that either have novalue
or unknown.

Change-Id: Ica5c4185abb5d15edb8571fdcb64cd4d90b07800
Signed-off-by: Loic Dachary <loic@dachary.org>