Make null + foo return foo, rather than an error.

This also allows 'add' to be implemented in jq rather than C.

testdata

@@ -205,6 +205,18 @@ null
 15
 19.0
 
+.+null
+{"a":42}
+{"a":42}
+
+null+.
+null
+null
+
+.a+.b
+{"a":42}
+42
+
 [1,2,3] + [.]
 null
 [1,2,3,null]