[Telemetry] enable default service config if no config from DB

Fix issue 16533, telemetry service exit in master and 202305 branches.

Enable default config if no TELEMETRY configs from redis DB.

After the fix, telemetry service would work with the following two
scenarios:
1. With TELEMETRY config in redis DB, load service configs from DB.
2. No TELEMETRY config in redis DB, use default service configs.

Author: qnos

dockers/docker-sonic-telemetry/telemetry.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 
 EXIT_TELEMETRY_VARS_FILE_NOT_FOUND=1
-INCORRECT_TELEMETRY_VALUE = 2
+INCORRECT_TELEMETRY_VALUE=2
 TELEMETRY_VARS_FILE=/usr/share/sonic/templates/telemetry_vars.j2
 
 if [ ! -f "$TELEMETRY_VARS_FILE" ]; then
@@ -75,7 +75,7 @@ THRESHOLD_CONNECTIONS=$(echo $GNMI | jq -r '.threshold')
 if [[ $THRESHOLD_CONNECTIONS =~ ^[0-9]+$ ]]; then
     TELEMETRY_ARGS+=" --threshold $THRESHOLD_CONNECTIONS"
 else
-    if [[ $THRESHOLD_CONNECTIONS == "null" ]]; then
+    if [ -z $GNMI ] || [[ $THRESHOLD_CONNECTIONS == "null" ]]; then
         TELEMETRY_ARGS+=" --threshold 100"
     else
         echo "Incorrect threshold value, expecting positive integers" >&2
@@ -88,7 +88,7 @@ IDLE_CONN_DURATION=$(echo $GNMI | jq -r '.idle_conn_duration')
 if [[ $IDLE_CONN_DURATION =~ ^[0-9]+$ ]]; then
     TELEMETRY_ARGS+=" --idle_conn_duration $IDLE_CONN_DURATION"
 else
-    if [[ $IDLE_CONN_DURATION == "null" ]]; then
+    if [ -z $GNMI ] || [[ $IDLE_CONN_DURATION == "null" ]]; then
         TELEMETRY_ARGS+=" --idle_conn_duration 5"
     else
         echo "Incorrect idle_conn_duration value, expecting positive integers" >&2