Use null-coalescing compound assignment for caches. (#295)

I didn't know this was possible before. It's neater so let's use it consistenly.

source/GenericData/GenericDataList.cs

@@ -30,7 +30,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.GenericData
                     : result;
         }
 
-        public static GenericDataList Empty => s_empty ?? (s_empty = new GenericDataList(Array.Empty<GenericDataObject>()));
+        public static GenericDataList Empty => s_empty ??= new GenericDataList(Array.Empty<GenericDataObject>());
 
         public GenericDataObject this[int index] => _items[index];
 

source/GenericData/GenericDataMap.cs

@@ -26,7 +26,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.GenericData
                 ? Empty
                 : new GenericDataMap(items);
 
-        public static GenericDataMap Empty => s_empty ?? (s_empty = new GenericDataMap(new Dictionary<string, GenericDataObject>(0)));
+        public static GenericDataMap Empty => s_empty ??= new GenericDataMap(new Dictionary<string, GenericDataObject>(0));
 
         public GenericDataObject this[string key] => _items[key];
 

source/LottieToWinComp/LottieToWinCompTranslator.cs

@@ -4579,7 +4579,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.LottieToWinComp
         void EnsureColorThemePropertyExists(TranslationContext context, string bindingName, Color defaultValue)
         {
             // Create a theme property set if one hasn't been created yet.
-            var themeProperties = _themePropertySet ?? (_themePropertySet = _c.CreatePropertySet());
+            var themeProperties = _themePropertySet ??= _c.CreatePropertySet();
 
             var defaultValueAsWinUIColor = Color(defaultValue);
             var defaultValueAsVector4 = Vector4(defaultValueAsWinUIColor);
@@ -4618,7 +4618,7 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.LottieToWinComp
         void EnsureScalarThemePropertyExists(string bindingName, double defaultValue)
         {
             // Create a theme property set if one hasn't been created yet.
-            var themeProperties = _themePropertySet ?? (_themePropertySet = _c.CreatePropertySet());
+            var themeProperties = _themePropertySet ??= _c.CreatePropertySet();
 
             var defaultValueAsFloat = Float(defaultValue);
 

source/WinCompData/Expressions/Expression_.cs

@@ -23,13 +23,12 @@ namespace Microsoft.Toolkit.Uwp.UI.Lottie.WinCompData.Expressions
         /// Gets a simplified form of the expression. May be the same as this.
         /// </summary>
         // Expressions are immutable, so it's always safe to return a cached version.
-        public T Simplified => _simplifiedExpressionCache ?? (_simplifiedExpressionCache = Simplify());
+        public T Simplified => _simplifiedExpressionCache ??= Simplify();
 
         /// <inheritdoc/>
         // Expressions are immutable, so it's always safe to return a cached version.
         // Always return the simplified version.
-        public override string ToText()
-            => _expressionTextCache ?? (_expressionTextCache = Simplified.CreateExpressionText());
+        public override string ToText() => _expressionTextCache ??= Simplified.CreateExpressionText();
 
         protected virtual T Simplify() => (T)this;