Refactoring of PR assimp#6092 (assimp#6101)

* Fix fbx export. Nodes "LayerElementNormal" and "LayerElementColor" should be written only when actual data are exported.

* Update FBXExporter.cpp

- Fix intentions
- use empty instead of size for checking data exists

* refactoring

---------

Co-authored-by: Kim Kulling <[email protected]>

Author: Riv1s-sSsA01

code/AssetLib/FBX/FBXExporter.cpp

@@ -1333,14 +1333,14 @@ void FBXExporter::WriteObjects () {
         layer.AddChild("Version", int32_t(100));
         FBX::Node le;
 
-		if (normal_data.size()) {
+		if (!normal_data.empty()) {
 		  le = FBX::Node("LayerElement");
 		  le.AddChild("Type", "LayerElementNormal");
 		  le.AddChild("TypedIndex", int32_t(0));
 		  layer.AddChild(le);
         }
 
-		if (color_data.size()) {
+		if (!color_data.empty()) {
 		  le = FBX::Node("LayerElement");
 		  le.AddChild("Type", "LayerElementColor");
 		  le.AddChild("TypedIndex", int32_t(0));