Fix fix for empty FLOW_CONTEXT in Local Beach

In (recent) Local Beach projects, `FLOW_CONTEXT` is set to an empty
value if not given in the environment. This lead to `FLOW_CONTEXT`
being empty, breaking configuration of Flow.

This fixes replacament of the check for being set with a check for
not being empty to fix this by also checking for the variable to be
set.

Otherwise it resulted in `FLOW_CONTEXT: unbound variable` during
startup.

Author: kdambekalns

root-files/opt/flownative/lib/beach.sh

@@ -34,10 +34,10 @@ export BEACH_PROJECT_NAME=${BEACH_PROJECT_NAME:-}
 export BEACH_FLOW_BASE_CONTEXT=${BEACH_FLOW_BASE_CONTEXT:-Production}
 export BEACH_FLOW_SUB_CONTEXT=${BEACH_FLOW_SUB_CONTEXT:-Instance}
 export BEACH_FLOW_CONTEXT=${BEACH_FLOW_BASE_CONTEXT}/Beach/${BEACH_FLOW_SUB_CONTEXT}
-if [[ -z "$FLOW_CONTEXT" ]]; then
-    export FLOW_CONTEXT=${BEACH_FLOW_CONTEXT}
-else
+if [[ -v FLOW_CONTEXT && ! -z "$FLOW_CONTEXT" ]]; then
     export FLOW_CONTEXT
+else
+    export FLOW_CONTEXT=${BEACH_FLOW_CONTEXT}
 fi
 
 export BEACH_DATABASE_HOST=${BEACH_DATABASE_HOST:-}