bump-formula-pr: fix when no 'version' stanza in formula found
bump-formula-pr: fix when no 'version' stanza in formula found

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.

konqui/brew,mahori/brew,MikeMcQuaid/brew,JCount/brew,vitorgalvao/brew,konqui/brew,EricFromCanada/brew,reitermarkus/brew,reitermarkus/brew,mahori/brew,Homebrew/brew,MikeMcQuaid/brew,nandub/brew,EricFromCanada/brew,reitermarkus/brew,claui/brew,sjackman/homebrew,vitorgalvao/brew,JCount/brew,claui/brew,EricFromCanada/brew,reitermarkus/brew,vitorgalvao/brew,JCount/brew,claui/brew,sjackman/homebrew,Homebrew/brew,sjackman/homebrew,vitorgalvao/brew,Homebrew/brew,nandub/brew,Homebrew/brew,claui/brew,MikeMcQuaid/brew,konqui/brew,MikeMcQuaid/brew,nandub/brew,sjackman/homebrew,konqui/brew,EricFromCanada/brew,mahori/brew,JCount/brew,nandub/brew,mahori/brew