Check isnothing(path) earlier in source_url
Check isnothing(path) earlier in source_url

If we are not in a Git repo, then path == nothing, and repo_commit will
throw. This patch makes it consistent with the 0.27 behavior where
isnothing(path) is checked before the repo_commit() call.

mortenpi/Documenter.jl,JuliaDocs/Documenter.jl