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/include_test/sub/include_test2.fbs

@@ -5,7 +5,7 @@ namespace MyGame.OtherNameSpace;
 
 enum FromInclude:long { IncludeVal }
 
-struct Unused {}
+struct Unused { a:int; }
 
 table TableB {
   a:TableA;

tests/monster_test.schema.json

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

tests/test.cpp

@@ -1230,6 +1230,7 @@ void ErrorTest() {
   TestError("enum X:byte (bit_flags) { Y=8 }", "bit flag out");
   TestError("table X { Y:int; } table X {", "datatype already");
   TestError("struct X (force_align: 7) { Y:int; }", "force_align");
+  TestError("struct X {}", "size 0");
   TestError("{}", "no root");
   TestError("table X { Y:byte; } root_type X; { Y:1 } { Y:1 }", "end of file");
   TestError("table X { Y:byte; } root_type X; { Y:1 } table Y{ Z:int }",