code-block: Handle 0 as numeric argument in :dedent:

Due to how Python converts ints to bool the numeric argument ``0`` to
the :dedent: option of the code-block directive was handled like if no
argument was given to :dedent:.

Check properly for None in the options argument processing, so ``0`` is
treated as numeric argument to :dedent:.

Bug: sphinx-doc#9636

CHANGES

@@ -23,6 +23,7 @@ Bugs fixed
 * #9644: autodoc: Crashed on getting source info from problematic object
 * #9630: autosummary: Failed to build summary table if :confval:`primary_domain`
   is not 'py'
+* #9636: code-block: ``:dedent:`` handled numeric argument ``0`` like no argument was given
 
 Testing
 --------

sphinx/directives/code.py

@@ -57,7 +57,7 @@ def run(self) -> List[Node]:
 
 
 def dedent_lines(lines: List[str], dedent: int, location: Tuple[str, int] = None) -> List[str]:
-    if not dedent:
+    if dedent is None:
         return textwrap.dedent(''.join(lines)).splitlines(True)
 
     if any(s[:dedent].strip() for s in lines):