In #2721, the ability for key-only values was added via using a
wrapperspb.StringValue that could be null. This introduced a bug because
originally the default case for `-attr` was to get the string value but
instead it used the incoming value as-is to allow for nulls; the null
check was moved up later but the original behavior was not restored. As
a result, non-explicitly-quoted strings carried through the
wrapperspb.StringValue, resulting in a map for the value in JSON instead
of a string.
This also fixes the ability to set explicitly quoted strings.
Jeff Mitchell
b286060ea8