Made codegen always output a file, even on an empty schema.

Previously, we had a check to simply skip such files, but this
tends to make build systems unhappy.

This only affects C++ and JS, since other language output per-class
files.

Change-Id: I54224642725bbafb9f6e1654ed3693e62ca9f7d7
Tested: on Linux.

src/idl_gen_js.cpp

@@ -83,8 +83,6 @@ class JsGenerator : public BaseGenerator {
   // Iterate through all definitions we haven't generate code for (enums,
   // structs, and tables) and output them to a single file.
   bool generate() {
-    if (IsEverythingGenerated()) return true;
-
     imported_fileset imported_files;
     reexport_map reexports;