[11.x] Fix etag headers for binary file responses

[wouterrutgers]

The getContent method always returns false for instances of BinaryFileResponse or StreamedResponse. Consequently, for any file served via return response()->download(...), the ETag header wil be set to the MD5 hash of false, which is d41d8cd98f00b204e9800998ecf8427e.

This leads to an identical ETag for all downloads, regardless of the actual file content. I propose that the ETag header be removed in cases where the content cannot be determined, as this would provide a more accurate response.