Add --force-empty-vectors option (#5653)

The rationale for this option is that JSON clients typically want empty arrays (i.e [] in the JSON) instead of missing properties, but not empty strings when the value isn't set.
--force-empty is kept as-is, i.e. it will force both empty strings and vectors.

Closes #5652

docs/source/Compiler.md

@@ -203,5 +203,8 @@ Additional options:
 -   `--force-empty` : When serializing from object API representation, force
      strings and vectors to empty rather than null.
 
+-   `--force-empty-vectors` : When serializing from object API representation, force
+     vectors to empty rather than null.
+
 NOTE: short-form options for generators are deprecated, use the long form
 whenever possible.

include/flatbuffers/idl.h

@@ -588,9 +588,13 @@ struct IDLOptions {
   // for code generation.
   unsigned long lang_to_generate;
 
-  // If set (default behavior), empty string and vector fields will be set to
-  // nullptr to make the flatbuffer more compact.
-  bool set_empty_to_null;
+  // If set (default behavior), empty string fields will be set to nullptr to make
+  // the flatbuffer more compact.
+  bool set_empty_strings_to_null;
+
+  // If set (default behavior), empty vector fields will be set to nullptr to make
+  // the flatbuffer more compact.
+  bool set_empty_vectors_to_null;
 
   IDLOptions()
       : use_flexbuffers(false),
@@ -635,7 +639,8 @@ struct IDLOptions {
         lang(IDLOptions::kJava),
         mini_reflect(IDLOptions::kNone),
         lang_to_generate(0),
-        set_empty_to_null(true) {}
+        set_empty_strings_to_null(true),
+        set_empty_vectors_to_null(true) {}
 };
 
 // This encapsulates where the parser is in the current source file.

src/idl_gen_cpp.cpp

@@ -2452,10 +2452,10 @@ class CppGenerator : public BaseGenerator {
 
         // For optional fields, check to see if there actually is any data
         // in _o->field before attempting to access it. If there isn't,
-        // depending on set_empty_to_null either set it to 0 or an empty string.
+        // depending on set_empty_strings_to_null either set it to 0 or an empty string.
         if (!field.required) {
           auto empty_value =
-              opts.set_empty_to_null ? "0" : "_fbb.CreateSharedString(\"\")";
+              opts.set_empty_strings_to_null ? "0" : "_fbb.CreateSharedString(\"\")";
           code = value + ".empty() ? " + empty_value + " : " + code;
         }
         break;
@@ -2557,10 +2557,10 @@ class CppGenerator : public BaseGenerator {
           }
         }
 
-        // If set_empty_to_null option is enabled, for optional fields, check to
+        // If set_empty_vectors_to_null option is enabled, for optional fields, check to
         // see if there actually is any data in _o->field before attempting to
         // access it.
-        if (opts.set_empty_to_null && !field.required) {
+        if (opts.set_empty_vectors_to_null && !field.required) {
           code = value + ".size() ? " + code + " : 0";
         }
         break;