[Tests] Handle empty git hash

The app directory isn't marked as safe => a test fails for accessing nil
Not sure how there's only 1 test failing because of this.
Either way, this is more correct

app/logical/git_helper.rb

@@ -5,7 +5,7 @@ module GitHelper
     elsif system("type git > /dev/null && git rev-parse --show-toplevel > /dev/null")
       @hash = `git rev-parse HEAD`.strip
     else
-      @hash = nil
+      @hash = ""
     end
   end