Without this fix, a command like:
`brew bump-formula-pr --version=0.3.3 --tag=v0.3.3 --revision=... test-formula-git-revision`
will fail because:
```
Error: no implicit conversion of nil into String
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/dev-cmd/bump-formula-pr.rb:281:in `escape'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/dev-cmd/bump-formula-pr.rb:281:in `bump_formula_pr'
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/brew.rb:111:in `<main>'
```
Additionally, reject all `nil` replacement pairs by calling `compact`
method.