Have tap_git_head return nil when no tap is installed

[Rylan12]

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Fixes Homebrew/discussions#5493

When the API is not being used but the core/cask taps aren't installed, tap_git_head will error on homebrew/core and homebrew/cask. This PR catches that error and returns nil. I think this should be fine since I scanned the code and it seemed like there wasn't much that should break, but it is posisble that I missed something.

Also, if desired, we could scope this to homebrew/core and homebrew/cask

[Bo98]

Thanks!