[Go] Write required string fields into the buffer when using Object API

In C++, CreateX allows to write the default "" value of a required
string, when the string is not explicitly set. Object API Pack method
uses this implementation of CreateX.

However, in go, despite whether the field is optional or required, it is
always checked against empty string allowing to create messages that
fail to pass the verifier. This commits partially reverts google#7719 when the
string field is required.

src/idl_gen_go.cpp

@@ -1111,11 +1111,16 @@ class GoGenerator : public BaseGenerator {
       const std::string offset = field_var + "Offset";
 
       if (IsString(field.value.type)) {
-        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";
+        if (!field.IsRequired()) {
+          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 {
+          code += "\t" + offset + " := builder.CreateString(t." + field_field +
+                  ")\n";
+        }
       } else if (IsVector(field.value.type) &&
                  field.value.type.element == BASE_TYPE_UCHAR &&
                  field.value.type.enum_def == nullptr) {