Fix false negative in regex subset

If s2 is a regex and s1 is empty or None, then s1 is equivalent to
all the  strings, but s2 can also be a regex equivalent to all the
strings (for instance `.*|[abc]`).

In that case, the two regex are equivalent and subsets of each other.

jsonsubschema/_utils.py

@@ -199,7 +199,7 @@ def regex_isSubset(s1, s2):
     elif s1:
         return True
     elif s2:
-        return False
+        return parse(s2).equivalent(parse(".*"))
 
 
 # def regex_isProperSubset(s1, s2):