A song is a song, no matter how you go about completing it. There aren't any fixed rules about songwriting.
There is no rule-book that states that the lyrics need to come first, or the melody, or for that matter the rhythm.
The point is to start in the mode that you're most comfortable with, if you're better at composing tunes than penning lyrics, then start with the melody first!
So why don't you challenge yourself today! Pick up an idea and work it up into a song.