Handle unset variables in various parts of the codebase (#8944)

DISABLE_UNTRACKED_FILES_DIRTY, DISABLE_AUTO_TITLE, GIT_STATUS_IGNORE_SUBMODULES are not set
Handle these variables not being set with conditional access.

If the user has set -u option to report attempts to use undeclared / unassigned variable, accessing the variables needs to be conditional.

lib/git.zsh

@@ -14,10 +14,10 @@ function parse_git_dirty() {
   local -a FLAGS
   FLAGS=('--porcelain')
   if [[ "$(command git config --get oh-my-zsh.hide-dirty)" != "1" ]]; then
-    if [[ "$DISABLE_UNTRACKED_FILES_DIRTY" == "true" ]]; then
+    if [[ "${DISABLE_UNTRACKED_FILES_DIRTY:-}" == "true" ]]; then
       FLAGS+='--untracked-files=no'
     fi
-    case "$GIT_STATUS_IGNORE_SUBMODULES" in
+    case "${GIT_STATUS_IGNORE_SUBMODULES:-}" in
       git)
         # let git decide (this respects per-repo config in .gitmodules)
         ;;

lib/termsupport.zsh

@@ -50,13 +50,13 @@ fi
 
 # Runs before showing the prompt
 function omz_termsupport_precmd {
-  [[ "$DISABLE_AUTO_TITLE" == true ]] && return
+  [[ "${DISABLE_AUTO_TITLE:-}" == true ]] && return
   title $ZSH_THEME_TERM_TAB_TITLE_IDLE $ZSH_THEME_TERM_TITLE_IDLE
 }
 
 # Runs before executing the command
 function omz_termsupport_preexec {
-  [[ "$DISABLE_AUTO_TITLE" == true ]] && return
+  [[ "${DISABLE_AUTO_TITLE:-}" == true ]] && return
 
   emulate -L zsh
   setopt extended_glob