Handle nil buffer in Reader and update tests

Updated the Reader struct in reader.go to properly handle nil buffers. This prevents potential errors from nil pointer dereference. Also added the fmt package for debugging purposes. Correspondingly, additional tests in reader_test.go were also added to cover these edge cases and ensure the Reader struct correctly handles them.

reader.go

@@ -25,7 +25,7 @@ func (r *Reader) Read(p []byte) (n int, err error) {
 	if r.err != nil {
 		return 0, r.err
 	}
-	if r.empty() {
+	if r.empty() || r.buf == nil {
 		r.Reset()
 		if len(p) == 0 {
 			return 0, nil

reader_test.go

@@ -6,7 +6,9 @@ package mail
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
+	"io"
 	"testing"
 )
 
@@ -64,9 +66,20 @@ func TestReader_Read_error(t *testing.T) {
 // TestReader_Read_empty tests the Reader.Read method with an empty buffer
 func TestReader_Read_empty(t *testing.T) {
 	r := Reader{buf: []byte{}}
-	var p []byte
+	p := make([]byte, 1)
+	p[0] = 'a'
 	_, err := r.Read(p)
-	if err != nil {
+	if err != nil && !errors.Is(err, io.EOF) {
+		t.Errorf("Reader failed: %s", err)
+	}
+}
+
+// TestReader_Read_nil tests the Reader.Read method with a nil buffer
+func TestReader_Read_nil(t *testing.T) {
+	r := Reader{buf: nil, off: -10}
+	p := make([]byte, 0)
+	_, err := r.Read(p)
+	if err != nil && !errors.Is(err, io.EOF) {
 		t.Errorf("Reader failed: %s", err)
 	}
 }