hook env: don't error if POD_UNIQUE_KEY is empty

[mpuncel]

POD_UNIQUE_KEY is empty for most pods, so it doesn't make sense to
return an error if it's empty, just return an empty string

[mpuncel]

hmm errcheck isn't failing locally for me

[spenceral]

I'm having the same errcheck issue on my PR (and similarly, it works locally)

[esuen]

In order to fix the errcheck issue you need to add something to your Rakefile, I should have already applied this change to the master branch.

Your errcheck in Rakefile should look like

task :errcheck do
  e "exit $(errcheck -ignoretests -ignore \"fmt:.*\" github.com/square/p2/pkg/... | grep -v defer | wc -l || 1)"
end

This is because errcheck was updated to selectively ignore functions in thefmt package, where the old behavior was to ignore everything in fmt.