`Filesystem::mimeType` return type conflicts with `FileAttributes::mimeType()`

[jacekkarczmarczyk]

Bug Report

Q	A
Flysystem Version	3.29.1
Adapter Name	-
Adapter version	-

Summary

FileAttributes::mimeType() returns ?string but Filesystem::mimeType() returns string. This means that if the former returns null, the latter will throw a TypeError exception. This issue may also affect other attributes.