gh-38847: Graph tikz method fixup
tikz method for graphs was recently added in #38798
default value for `format` should be `None` as documented.
### :hourglass: Dependencies
#38798
URL: https://github.com/sagemath/sage/pull/38847
Reported by: Sébastien Labbé
Reviewer(s): David Coudert, Frédéric Chapoton
