Replace 'is null' with more compatible clause and fix MSBuild bug. (#465)

- Replace is null usages in generated code with downlevel equivalent to support older versions of C# in client applications
- Fix MSBuild task to send bool args correctly. The LottieGen cmdline only allows flags to be set without an associated value arg.

Author: aschultz

LottieGen/MSBuildTask/LottieGen.cs

@@ -246,7 +246,7 @@ void AddOptionalBool(string parameterName, bool value)
             {
                 if (value)
                 {
-                    AddArg(parameterName, "true");
+                    args.Add($"-{parameterName}");
                 }
             }
 

source/UIDataCodeGen/CodeGen/CSharp/CSharpInstantiatorGenerator.cs

@@ -211,7 +211,7 @@ void WriteThemeProperties(CodeBuilder builder)
                     builder.WriteLine("set");
                     builder.OpenScope();
                     builder.WriteLine($"_theme{prop.BindingName} = value;");
-                    builder.WriteLine($"if ({SourceInfo.ThemePropertiesFieldName} is not null)");
+                    builder.WriteLine($"if ({SourceInfo.ThemePropertiesFieldName} != null)");
                     builder.OpenScope();
                     WriteThemePropertyInitialization(builder, SourceInfo.ThemePropertiesFieldName, prop);
                     builder.CloseScope();
@@ -408,7 +408,7 @@ void WriteThemeMethodsAndFields(CodeBuilder builder)
                 // EnsureThemeProperties(...) method implementation.
                 builder.WriteLine("CompositionPropertySet EnsureThemeProperties(Compositor compositor)");
                 builder.OpenScope();
-                builder.WriteLine($"if ({SourceInfo.ThemePropertiesFieldName} is null)");
+                builder.WriteLine($"if ({SourceInfo.ThemePropertiesFieldName} == null)");
                 builder.OpenScope();
                 builder.WriteLine($"{SourceInfo.ThemePropertiesFieldName} = compositor.CreatePropertySet();");
 
@@ -714,7 +714,7 @@ void WriteSetPropertyImpl(
                 builder.WriteLine();
 
                 // Update the CompositionPropertySet if it has been created.
-                builder.WriteLine($"if ({SourceInfo.ThemePropertiesFieldName} is not null)");
+                builder.WriteLine($"if ({SourceInfo.ThemePropertiesFieldName} != null)");
                 builder.OpenScope();
 
                 builder.WriteLine($"{SourceInfo.ThemePropertiesFieldName}.Insert{propertySetTypeName}(propertyName, {valueInitializer});");