Fix reading the number argument from config file (#923)

* Fix reading the number argument from config file

When passing the "number" option from the INI file we did
not take into account to store its value as an integer
(when that value is not None).

Resolves: #922

* Implement a cleaner fix

We use the "or" boolean operation to have a
cleaner implementation of the solution.

bandit/cli/main.py

@@ -512,7 +512,7 @@ def main():
         args.context_lines = _log_option_source(
             parser.get_default("context_lines"),
             args.context_lines,
-            ini_options.get("number"),
+            int(ini_options.get("number") or 0) or None,
             "max code lines output for issue",
         )