fix(chezmoi): Properly use default branch

`yq` outputs "null" string when no results are found, so it's not an empty string.

Branch "null" is passed as custom branch, which very likely doesn't exist in user's chezmoi repo.

So to solve this issue, set "if string is empty or contains string "null", disable custom branch, otherwise enable"

modules/chezmoi/chezmoi.sh

@@ -19,10 +19,10 @@ fi
 DOTFILE_REPOSITORY=$(echo "$1" | yq -I=0 ".repository") # (string)
 # The chezmoi repository branch to use.
 DOTFILE_BRANCH=$(echo "$1" | yq -I=0 ".branch")
-if [[ -n ${DOTFILE_BRANCH} ]]; then
-	INIT_BRANCH_FLAG="--branch ${DOTFILE_BRANCH}"
+if ! [[ -z "${DOTFILE_BRANCH}" ]] || [[ "${DOTFILE_BRANCH}" == "null" ]]; then
+  INIT_BRANCH_FLAG="--branch ${DOTFILE_BRANCH}"
 else
-	INIT_BRANCH_FLAG=""
+  INIT_BRANCH_FLAG=""
 fi
 
 # If true, chezmoi services will be enabled for all logged in users, and users with lingering enabled. (default: true)