Validate module metadata to be an object (#2357)

tests/jq.test

@@ -1495,6 +1495,10 @@ null
 module (.+1); 0
 jq: error: Module metadata must be constant at <top-level>, line 1:
 
+%%FAIL
+module []; 0
+jq: error: Module metadata must be an object at <top-level>, line 1:
+
 %%FAIL
 include "a" (.+1); 0
 jq: error: Module metadata must be constant at <top-level>, line 1: