Go optional scalars (#7104)

* [go] always write required types

* support optional scalars in go

* generate optional_scalars and monster_test

* restore original behavior for non-optional scalars

* add tests

tests/optional_scalars/OptionalByte.go

@@ -0,0 +1,32 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package optional_scalars
+
+import "strconv"
+
+type OptionalByte int8
+
+const (
+	OptionalByteNone OptionalByte = 0
+	OptionalByteOne  OptionalByte = 1
+	OptionalByteTwo  OptionalByte = 2
+)
+
+var EnumNamesOptionalByte = map[OptionalByte]string{
+	OptionalByteNone: "None",
+	OptionalByteOne:  "One",
+	OptionalByteTwo:  "Two",
+}
+
+var EnumValuesOptionalByte = map[string]OptionalByte{
+	"None": OptionalByteNone,
+	"One":  OptionalByteOne,
+	"Two":  OptionalByteTwo,
+}
+
+func (v OptionalByte) String() string {
+	if s, ok := EnumNamesOptionalByte[v]; ok {
+		return s
+	}
+	return "OptionalByte(" + strconv.FormatInt(int64(v), 10) + ")"
+}