Fix always-true conditional check in SpannerDataProviderService (#986)

Co-authored-by: marcopremier <[email protected]>

src/main/kotlin/org/wfanet/measurement/kingdom/deploy/gcloud/spanner/SpannerDataProvidersService.kt

@@ -54,9 +54,7 @@ class SpannerDataProvidersService(
   override suspend fun replaceDataProviderRequiredDuchies(
     request: ReplaceDataProviderRequiredDuchiesRequest
   ): DataProvider {
-    grpcRequire(request.externalDataProviderId != null) {
-      "externalDataProviderId field is missing."
-    }
+    grpcRequire(request.externalDataProviderId != 0L) { "externalDataProviderId field is missing." }
     try {
       return ReplaceDataProviderRequiredDuchies(request).execute(client, idGenerator)
     } catch (e: DataProviderNotFoundException) {