Guard against undefined variables when PS1 is evaluated (#1020)

* Guard against undefined variables when PS1 is evaluated

* Update version in devcontainer-feature.json

src/common-utils/devcontainer-feature.json

 {
     "id": "common-utils",
-    "version": "2.4.3",
+    "version": "2.4.4",
     "name": "Common Utilities",
     "documentationURL": "https://github.com/devcontainers/features/tree/main/src/common-utils",
     "description": "Installs a set of common command line utilities, Oh My Zsh!, and sets up a non-root user.",
@@ -66,4 +66,4 @@
             "description": "Add packages from non-free Debian repository? (Debian only)"
         }
     }
-}
\ No newline at end of file
+}

src/common-utils/scripts/bash_theme_snippet.sh

@@ -2,13 +2,13 @@
 # bash theme - partly inspired by https://github.com/ohmyzsh/ohmyzsh/blob/master/themes/robbyrussell.zsh-theme
 __bash_prompt() {
     local userpart='`export XIT=$? \
-        && [ ! -z "${GITHUB_USER}" ] && echo -n "\[\033[0;32m\]@${GITHUB_USER} " || echo -n "\[\033[0;32m\]\u " \
+        && [ ! -z "${GITHUB_USER:-}" ] && echo -n "\[\033[0;32m\]@${GITHUB_USER:-} " || echo -n "\[\033[0;32m\]\u " \
         && [ "$XIT" -ne "0" ] && echo -n "\[\033[1;31m\]➜" || echo -n "\[\033[0m\]➜"`'
     local gitbranch='`\
         if [ "$(git config --get devcontainers-theme.hide-status 2>/dev/null)" != 1 ] && [ "$(git config --get codespaces-theme.hide-status 2>/dev/null)" != 1 ]; then \
-            export BRANCH=$(git --no-optional-locks symbolic-ref --short HEAD 2>/dev/null || git --no-optional-locks rev-parse --short HEAD 2>/dev/null); \
-            if [ "${BRANCH}" != "" ]; then \
-                echo -n "\[\033[0;36m\](\[\033[1;31m\]${BRANCH}" \
+            export BRANCH="$(git --no-optional-locks symbolic-ref --short HEAD 2>/dev/null || git --no-optional-locks rev-parse --short HEAD 2>/dev/null)"; \
+            if [ "${BRANCH:-}" != "" ]; then \
+                echo -n "\[\033[0;36m\](\[\033[1;31m\]${BRANCH:-}" \
                 && if [ "$(git config --get devcontainers-theme.show-dirty 2>/dev/null)" = 1 ] && \
                     git --no-optional-locks ls-files --error-unmatch -m --directory --no-empty-directory -o --exclude-standard ":/*" > /dev/null 2>&1; then \
                         echo -n " \[\033[1;33m\]✗"; \