Fix crash on pyproject.toml without bandit config

This is a naive fix for bandit crashing when it encounters a
`pyproject.toml` which does not contain any specific bandit
configuration.

This resolves the common failure mode that is seen, but does not cause
bandit to fall back to another configuration source if the
`pyproject.toml` does not contain any `tool.bandit` block.

Resolves PyCQA#1027

bandit/core/config.py

@@ -52,7 +52,7 @@ def __init__(self, config_file=None):
 
                 try:
                     with f:
-                        self._config = tomllib.load(f)["tool"]["bandit"]
+                        self._config = tomllib.load(f).get("tool", {}).get("bandit", {})
                 except tomllib.TOMLDecodeError as err:
                     LOG.error(err)
                     raise utils.ConfigError("Error parsing file.", config_file)