update default saml secret value

chart/values.yaml

@@ -296,7 +296,7 @@ anchoreGlobal:
   saml:
     # Locations for keys used for signing and encryption. Only one of 'secret' or 'privateKeyName'/'publicKeyName' needs to be set. If all are set then the keys take precedence over the secret value
     # Secret is for a shared secret and if set, all components in anchore should have the exact same value in their configs.
-    secret: Null
+    secret: ""
     # If set to true, use the secret specified in anchoreGlobal.existingSecret to set the ANCHORE_SAML_SECRET env variable
     useExistingSecret: false
     privateKeyName: Null