Skip empty lines when determining base indentation (#9795)
## Summary
It turns out we saw a panic in cases when dedenting blocks like the `def
wrapper` here:
```python
def instrument_url(f: UrlFuncT) -> UrlFuncT:
# TODO: Type this with ParamSpec to preserve the function signature.
if not INSTRUMENTING: # nocoverage -- option is always enabled; should we remove?
return f
else:
def wrapper(
self: "ZulipTestCase", url: str, info: object = {}, **kwargs: Union[bool, str]
) -> HttpResponseBase:
```
Since we relied on the first line to determine the indentation, instead
of the first non-empty line.
## Test Plan
`cargo test`
