Do not crash when requesting sigil from a directory instead of a file

[Morriar]

Requesting the sigil from a path that points to a directory was ending up in a crash:

Sigils.file_strictness("/")

Errno::EISDIR: Is a directory @ io_fread - /
    lib/spoom/sorbet/sigils.rb:59:in `read'
    lib/spoom/sorbet/sigils.rb:59:in `file_strictness'

With this PR we return nil instead.