Disallowed 0 byte structs.

This is because they are incompatible with C++ and possibly other
languages that make them minimum size 1 (to make sure multiple
such objects don't reside at the same address). Forcing them to size
1 was also not practical, as that is requires updating the logic
of a lot of implementations and thus possibly backwards incompatible.

More here: https://github.com/google/flatbuffers/issues/4122

Change-Id: I2bfdc8597b7cfd2235bb4074bb2ae06f81f8e57d

tests/monster_test.schema.json

@@ -16,6 +16,7 @@
     "MyGame_OtherNameSpace_Unused" : {
       "type" : "object",
       "properties" : {
+        "a" : { "type" : "number" }
       },
       "additionalProperties" : false
     },