fix(deva): guard set_user_env_value against empty retained array

USER_ENVS=("${retained[@]}") crashes under set -u when retained
is empty (all existing entries filtered out). Use explicit length
check to assign empty array safely.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: lroolle

deva.sh

@@ -554,7 +554,11 @@ set_user_env_value() {
         retained+=("$spec")
     done
 
-    USER_ENVS=("${retained[@]}")
+    if [ ${#retained[@]} -gt 0 ]; then
+        USER_ENVS=("${retained[@]}")
+    else
+        USER_ENVS=()
+    fi
     USER_ENVS+=("$name=$value")
     export "$name=$value"
 }