fix #8473

[`find_source_file`](https://github.com/JuliaLang/julia/blob/02546dea4ebb51a9b2f0872a3d8b23998a0b2cfd/base/loading.jl#L27) has a `Union(String, Nothing)` return type. In this case, the performance does not matter, but it seems like a pattern that we usually discourage anyway. Any suggestions?