SemanticsProperties: default `identifier` and `tooltip` to null (#140283)

This PR applies [the suggestion made here](https://github.com/flutter/devtools/pull/6942#issuecomment-1852773200).

packages/flutter/lib/src/semantics/semantics.dart

@@ -1656,7 +1656,7 @@ class SemanticsProperties extends DiagnosticableTree {
     properties.add(DiagnosticsProperty<bool>('mixed', mixed, defaultValue: null));
     properties.add(DiagnosticsProperty<bool>('expanded', expanded, defaultValue: null));
     properties.add(DiagnosticsProperty<bool>('selected', selected, defaultValue: null));
-    properties.add(StringProperty('identifier', identifier));
+    properties.add(StringProperty('identifier', identifier, defaultValue: null));
     properties.add(StringProperty('label', label, defaultValue: null));
     properties.add(AttributedStringProperty('attributedLabel', attributedLabel, defaultValue: null));
     properties.add(StringProperty('value', value, defaultValue: null));
@@ -1667,7 +1667,7 @@ class SemanticsProperties extends DiagnosticableTree {
     properties.add(AttributedStringProperty('attributedDecreasedValue', attributedDecreasedValue, defaultValue: null));
     properties.add(StringProperty('hint', hint, defaultValue: null));
     properties.add(AttributedStringProperty('attributedHint', attributedHint, defaultValue: null));
-    properties.add(StringProperty('tooltip', tooltip));
+    properties.add(StringProperty('tooltip', tooltip, defaultValue: null));
     properties.add(EnumProperty<TextDirection>('textDirection', textDirection, defaultValue: null));
     properties.add(DiagnosticsProperty<SemanticsSortKey>('sortKey', sortKey, defaultValue: null));
     properties.add(DiagnosticsProperty<SemanticsHintOverrides>('hintOverrides', hintOverrides, defaultValue: null));

packages/flutter/test/semantics/semantics_update_test.dart

@@ -156,11 +156,9 @@ void main() {
       'Semantics('
         'container: false, '
         'properties: SemanticsProperties, '
-        'identifier: null, '// ignore: missing_whitespace_between_adjacent_strings
         'attributedLabel: "label" [SpellOutStringAttribute(TextRange(start: 0, end: 5))], '
         'attributedValue: "value" [LocaleStringAttribute(TextRange(start: 0, end: 5), en-MX)], '
-        'attributedHint: "hint" [SpellOutStringAttribute(TextRange(start: 1, end: 2))], '
-        'tooltip: null'// ignore: missing_whitespace_between_adjacent_strings
+        'attributedHint: "hint" [SpellOutStringAttribute(TextRange(start: 1, end: 2))]'// ignore: missing_whitespace_between_adjacent_strings
       ')',
     );