Make LibraryReader.pathToUrl take a non-null argument

It is statically breaking to move the runtime null check to a static null check, but it's worth doing because a static check is preferred over a runtime check.

Locate LibraryReader.pathToUrl in the source_gen code and inspect its callers and current nullability checks to understand the API impact. Done means the argument is statically non-null and the affected code still passes static analysis; the issue does not mention a specific file or test.

Written by the indexing model from the issue text.