gh-92986: Fix ast.unparse when ImportFrom.level is None (#92992)

This doesn't happen naturally, but is allowed by the ASDL and compiler.
We don't want to change ASDL for backward compatibility reasons
(#57645, #92987)

Lib/ast.py

@@ -853,7 +853,7 @@ class _Unparser(NodeVisitor):
 
     def visit_ImportFrom(self, node):
         self.fill("from ")
-        self.write("." * node.level)
+        self.write("." * (node.level or 0))
         if node.module:
             self.write(node.module)
         self.write(" import ")