Fix application variable issue (#20500)

Fixes wrong formatting converting secret value to ******** before saving
Issued by
https://discord.com/channels/1130383047699738754/1423290797079662602/1502674904770936903

packages/twenty-server/src/engine/core-modules/logic-function/logic-function-executor/utils/build-env-var.ts

@@ -1,5 +1,6 @@
 import { type FlatApplicationVariable } from 'src/engine/metadata-modules/flat-application-variable/types/flat-application-variable.type';
 import { type SecretEncryptionService } from 'src/engine/core-modules/secret-encryption/secret-encryption.service';
+import { isNonEmptyString } from '@sniptt/guards';
 
 export const buildEnvVar = (
   flatApplicationVariables: FlatApplicationVariable[],
@@ -9,9 +10,10 @@ export const buildEnvVar = (
     (acc, flatApplicationVariable) => {
       const value = String(flatApplicationVariable.value ?? '');
 
-      acc[flatApplicationVariable.key] = flatApplicationVariable.isSecret
-        ? secretEncryptionService.decrypt(value)
-        : value;
+      acc[flatApplicationVariable.key] =
+        flatApplicationVariable.isSecret && isNonEmptyString(value)
+          ? secretEncryptionService.decrypt(value)
+          : value;
 
       return acc;
     },