Remove unnecessary null check in getPropertyValue

CSSStyleDeclaration.getPropertyValue returns a non-nullable string.
It will also return an empty string if the property is not found. Since
getPropertyValue returns the value of the native getPropertyValue, the
null check should be removed.

Change-Id: I9846553d3f9fcd68cbd15b9c39d2104cab594b7a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/136413
Reviewed-by: Sigmund Cherem <[email protected]>
Commit-Queue: Srujan Gaddam <[email protected]>

Author: srujzs

pkg/dev_compiler/tool/dart2js_nnbd_sdk_error_golden.txt

@@ -15,5 +15,4 @@ WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|1476|39|5|The left
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|8384|60|5|The left operand can't be null, so the right operand is never executed.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|9311|54|5|The left operand can't be null, so the right operand is never executed.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/developer/developer.dart|315|25|23|The left operand can't be null, so the right operand is never executed.
-WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/html/dart2js/html_dart2js.dart|4076|25|2|The left operand can't be null, so the right operand is never executed.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/io/io.dart|9167|16|1|The left operand can't be null, so the right operand is never executed.

pkg/dev_compiler/tool/dartdevc_nnbd_sdk_error_golden.txt

@@ -20,5 +20,4 @@ WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|8384|60|5|The left
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|9311|54|5|The left operand can't be null, so the right operand is never executed.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/collection/collection.dart|1076|46|13|The left operand can't be null, so the right operand is never executed.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/developer/developer.dart|332|25|23|The left operand can't be null, so the right operand is never executed.
-WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/html/dart2js/html_dart2js.dart|4076|25|2|The left operand can't be null, so the right operand is never executed.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/io/io.dart|9167|16|1|The left operand can't be null, so the right operand is never executed.

sdk/lib/html/dart2js/html_dart2js.dart

@@ -3685,8 +3685,7 @@ class CssStyleDeclaration extends Interceptor with CssStyleDeclarationBase {
   ///
   /// Please note the property name uses camelCase, not-hyphens.
   String getPropertyValue(String propertyName) {
-    var propValue = _getPropertyValueHelper(propertyName);
-    return propValue ?? '';
+    return _getPropertyValueHelper(propertyName);
   }
 
   String _getPropertyValueHelper(String propertyName) {

sdk_nnbd/lib/html/dart2js/html_dart2js.dart

@@ -4072,8 +4072,7 @@ class CssStyleDeclaration extends Interceptor with CssStyleDeclarationBase {
   ///
   /// Please note the property name uses camelCase, not-hyphens.
   String getPropertyValue(String propertyName) {
-    var propValue = _getPropertyValueHelper(propertyName);
-    return propValue ?? '';
+    return _getPropertyValueHelper(propertyName);
   }
 
   String _getPropertyValueHelper(String propertyName) {

tools/dom/scripts/css_code_generator.py

@@ -125,8 +125,7 @@ def GenerateCssTemplateFile():
   ///
   /// Please note the property name uses camelCase, not-hyphens.
   String getPropertyValue(String propertyName) {
-    var propValue = _getPropertyValueHelper(propertyName);
-    return propValue ?? '';
+    return _getPropertyValueHelper(propertyName);
   }
 
   String _getPropertyValueHelper(String propertyName) {

tools/dom/templates/html/impl/impl_CSSStyleDeclaration.darttemplate

@@ -27,8 +27,7 @@ $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME $EXTENDS with
   ///
   /// Please note the property name uses camelCase, not-hyphens.
   String getPropertyValue(String propertyName) {
-    var propValue = _getPropertyValueHelper(propertyName);
-    return propValue ?? '';
+    return _getPropertyValueHelper(propertyName);
   }
 
   String _getPropertyValueHelper(String propertyName) {