Add checks for nil or empty Subject headers

Added condition checks in `msg_test.go` for nil or empty Subject headers in the assigned functions. If the Subject is found to be nil or its length is less than 1, an empty string is now appended to prevent issues arising from null pointers or non-existent Subject headers in email messages.

msg_test.go

@@ -216,6 +216,9 @@ func (mw uppercaseMiddleware) Handle(m *Msg) *Msg {
 	if !ok {
 		fmt.Println("can't find the subject header")
 	}
+	if s == nil || len(s) < 1 {
+		s = append(s, "")
+	}
 	m.Subject(strings.ToUpper(s[0]))
 	return m
 }
@@ -231,6 +234,9 @@ func (mw encodeMiddleware) Handle(m *Msg) *Msg {
 	if !ok {
 		fmt.Println("can't find the subject header")
 	}
+	if s == nil || len(s) < 1 {
+		s = append(s, "")
+	}
 	m.Subject(strings.Replace(s[0], "a", "@", -1))
 	return m
 }