analyzer: fix analysis of empty section in analysis options

With a recent change, I noticed that I was getting a lot of "invalid
section" warnings, with valid YAML.

Change-Id: I7ee64fc793cb74d54e74d9ebeb21b350c5bb7584
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446563
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: srawlins

pkg/analyzer/lib/src/analysis_options/options_file_validator.dart

@@ -1077,9 +1077,9 @@ class _TopLevelOptionValidator extends OptionsValidator {
   @override
   void validate(DiagnosticReporter reporter, YamlMap options) {
     var node = options.valueAt(pluginName);
-    if (node == null) {
-      return;
-    }
+    if (node == null) return;
+    if (node is YamlScalar && node.value == null) return;
+
     if (node is! YamlMap) {
       reporter.atSourceSpan(
         node.span,

pkg/analyzer/test/src/options/options_file_validator_test.dart

@@ -143,6 +143,13 @@ analyzer:
     );
   }
 
+  test_analyzer_empty() {
+    registerLintRule(TestRule());
+    validate('''
+analyzer:
+''', []);
+  }
+
   test_analyzer_enableExperiment_badValue() {
     validate(
       '''