Handle incomplete rule options gracefully.

pkg/analysis_server/test/services/linter/linter_test.dart

Index: pkg/analysis_server/test/services/linter/linter_test.dart
diff --git a/pkg/analysis_server/test/services/linter/linter_test.dart b/pkg/analysis_server/test/services/linter/linter_test.dart
index a1f57ebc414efabef7812b976233367e0b50719f..00a9fd6abdc4af52ffff536764338061f671de27 100644
--- a/pkg/analysis_server/test/services/linter/linter_test.dart
+++ b/pkg/analysis_server/test/services/linter/linter_test.dart
@@ -21,7 +21,7 @@ main() {
 
 @reflectiveTest
 class LinterRuleOptionsValidatorTest {
-  final LinterRuleOptionsValidator validator= new LinterRuleOptionsValidator();
+  final LinterRuleOptionsValidator validator = new LinterRuleOptionsValidator();
   final AnalysisOptionsProvider optionsProvider = new AnalysisOptionsProvider();
 
   RecordingErrorListener recorder;
@@ -53,6 +53,17 @@ linter:
         []);
   }
 
+  test_linter_null_rule() {
+    validate(
+        '''
+linter:
+  rules:
+    -
+
+    ''',
+        []);
+  }
+
   test_linter_undefined_rule() {
     validate(
         '''