When passing {} or { encoding: 'utf8' } as options to readFile, the
flag is not defaulted to 'r' unlike normal fs. This fix makes
(fs/promises).readFile() act consistently with fs.readFile().

fail on unhandled rejection

common was required, but not assigned to common in order for the requested change to work.

fs: return string on empty fs.promises.readfile when encoding provided

fs.promises.readfile() would provide a buffer whenever an empty file
is read, when it should have returned an empty string when encoding
is provided.