fix: default optional env vars used with bash ,, in start.sh (#24683)

start.sh runs with `set -euo pipefail`, but three call sites added in
070ab2650 (refac: reorganize scripts and ci workflows) reference
optional env vars via bash's `,,` lowercase expansion without any
default. Containers that don't set these vars — the default for every
deployment that isn't explicitly opting into Playwright / bundled
Ollama / CUDA — crash on startup with:

    start.sh: line 15: WEB_LOADER_ENGINE: unbound variable

(and the same for USE_OLLAMA_DOCKER, USE_CUDA_DOCKER once the first
were set in turn.) Reported in open-webui#24560 by urbenlegend.

The same refactor correctly defaulted every other optional env var
with `${VAR:-…}`. The three `,,` references slipped through because
bash can't combine `:-default` with `,,` in a single substitution —
`${VAR:-default,,}` makes the default literal `,,`, not what's wanted.

Fix: normalise the three vars in a one-line preamble with `${VAR:=}`,
which assigns an empty default if unset. The downstream `${VAR,,}`
expressions stay exactly as Tim wrote them, preserving the file's
visual style and matching the existing `${VAR:-…}` idiom for "this
variable is optional".

backend/start.sh

@@ -7,6 +7,12 @@ set -euo pipefail
 # HuggingFace Space deployment, and launches the uvicorn server.
 # ---------------------------------------------------------------------------
 
+# Default optional env vars that we test below with bash's `,,` lowercase
+# expansion. The two can't be combined inline (`${VAR:-default,,}` makes
+# the default literal `,,`), so we normalise once up front and the simple
+# `${VAR,,}` form stays safe under `set -u` everywhere else.
+: "${WEB_LOADER_ENGINE:=}" "${USE_OLLAMA_DOCKER:=}" "${USE_CUDA_DOCKER:=}"
+
 SCRIPT_DIR=$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" &>/dev/null && pwd)
 cd "$SCRIPT_DIR" || exit 1