Perform nil check on string fields when packing (#7719)

Co-authored-by: Derek Bailey <derekbailey@google.com>

src/idl_gen_go.cpp

@@ -1054,8 +1054,11 @@ class GoGenerator : public BaseGenerator {
       const std::string offset = field_var + "Offset";
 
       if (IsString(field.value.type)) {
-        code +=
-            "\t" + offset + " := builder.CreateString(t." + field_field + ")\n";
+        code += "\t" + offset + " := flatbuffers.UOffsetT(0)\n";
+        code += "\tif t." + field_field + " != \"\" {\n";
+        code += "\t\t" + offset + " = builder.CreateString(t." + field_field +
+                ")\n";
+        code += "\t}\n";
       } else if (IsVector(field.value.type) &&
                  field.value.type.element == BASE_TYPE_UCHAR &&
                  field.value.type.enum_def == nullptr) {

tests/MyGame/Example/Monster.go

@@ -74,7 +74,10 @@ type MonsterT struct {
 
 func (t *MonsterT) Pack(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	if t == nil { return 0 }
-	nameOffset := builder.CreateString(t.Name)
+	nameOffset := flatbuffers.UOffsetT(0)
+	if t.Name != "" {
+		nameOffset = builder.CreateString(t.Name)
+	}
 	inventoryOffset := flatbuffers.UOffsetT(0)
 	if t.Inventory != nil {
 		inventoryOffset = builder.CreateByteString(t.Inventory)

tests/MyGame/Example/Stat.go

@@ -14,7 +14,10 @@ type StatT struct {
 
 func (t *StatT) Pack(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	if t == nil { return 0 }
-	idOffset := builder.CreateString(t.Id)
+	idOffset := flatbuffers.UOffsetT(0)
+	if t.Id != "" {
+		idOffset = builder.CreateString(t.Id)
+	}
 	StatStart(builder)
 	StatAddId(builder, idOffset)
 	StatAddVal(builder, t.Val)