Return nil on empty path

The execution path wasn't clear from the error, but either way this is a
fix.

Closes gitlab-org/gitaly#1115

lib/gitlab/git/commit.rb

@@ -486,6 +486,8 @@ module Gitlab
       end
 
       def tree_entry(path)
+        return unless path.present?
+
         @repository.gitaly_migrate(:commit_tree_entry) do |is_migrated|
           if is_migrated
             gitaly_tree_entry(path)

spec/models/commit_spec.rb

@@ -450,6 +450,11 @@ eos
       it "returns nil if the path doesn't exists" do
         expect(commit.uri_type('this/path/doesnt/exist')).to be_nil
       end
+
+      it 'is nil if the path is nil or empty' do
+        expect(commit.uri_type(nil)).to be_nil
+        expect(commit.uri_type("")).to be_nil
+      end
     end
 
     context 'when Gitaly commit_tree_entry feature is enabled' do