Accommodates find_pyproject_toml returning None

Prior to this, the return value of `black.files.find_pyproject_toml` was passed directly
to `os.path.exists`. This was fine where `black` could find a `pyproject.toml` file.
However, when one cannot be located, `find_pyproject_toml` returns `None`, which results
in an uncaught exception from `os.path.exists`. This adds an additional check for when
`find_project_toml` returns `None`.

Fixes #1955 (introduced by #1950).

elpy/blackutil.py

@@ -55,7 +55,7 @@ def fix_code(code, directory):
         pyproject_path = find_pyproject_toml((directory,))
     else:
         pyproject_path = os.path.join(directory, "pyproject.toml")
-    if toml is not None and os.path.exists(pyproject_path):
+    if toml and pyproject_path and os.path.exists(pyproject_path):
         pyproject_config = toml.load(pyproject_path)
         black_config = pyproject_config.get("tool", {}).get("black", {})
         if "line-length" in black_config: