Use the more specific checkNotNull vs checkState

Author: misolt

dumper/app/src/main/java/com/google/edwmigration/dumper/application/dumper/connector/Connector.java

@@ -61,8 +61,7 @@ static void validateDateRange(@Nonnull ConnectorArguments arguments) {
     if (start == null && end == null) {
       return;
     }
-    checkState(
-        start != null, "End date can be specified only with start date, but start date was null.");
+    checkNotNull(start, "End date can be specified only with start date, but start date was null.");
     // The assignment makes 'end' recognized as @Nonnull.
     end = checkNotNull(end, "End date must be specified with start date, but was null.");