Developers often want to create empty branches. They can't create it using GUI, because SourceTree can create branch only from working copy parent or specified commit.

To create empty branch, you have to go to terminal and execute:

git checkout --orphan branchname

git rm -rf .

add files

git add .

git commit -m "commit message"

git push -u origin branchname

Only after that you may see the new branch on GUI.

Please, add the feature for creating new empty (without parent) branches on GUI.