gh-38847: Graph tikz method fixup
    
tikz method for graphs was recently added in #38798

default value for `format` should be `None` as documented.

### ⌛ Dependencies

#38798
    
URL: #38847
Reported by: Sébastien Labbé
Reviewer(s): David Coudert, Frédéric Chapoton

Author: Release Manager

src/sage/graphs/generic_graph.py

@@ -939,7 +939,7 @@ def _latex_(self):
 
         return self.latex_options().latex()
 
-    def tikz(self, format='dot2tex', edge_labels=None,
+    def tikz(self, format=None, edge_labels=None,
             color_by_label=False, prog='dot', rankdir='down',
             standalone_config=None, usepackage=None,
             usetikzlibrary=None, macros=None,