fix: Return NULL on enum-value snprintf failure in GCS extra-data handler

handle_extra_data_enum() returns a `const nbgl_contentValueExt_t *`
but the failure path of its snprintf() guard returned `false`. C lets
the implicit bool→pointer conversion compile (0 → NULL), so the
behavior was accidentally correct, but the literal is misleading and
masks the intent of the early-return.

Return NULL explicitly to match the pointer return type.

(cherry picked from commit 38215d0b2875ff5b36dd915de9633759fe4cd1fa)