Address review: remove #nullable enable, use IsNullOrEmpty extension

- Remove redundant #nullable enable from 6 Generator files and task file
- Convert string.IsNullOrEmpty to IsNullOrEmpty extension in task

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: simonrozsival

src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/AcwMapWriter.cs

@@ -1,4 +1,3 @@
-#nullable enable
 using System;
 using System.Collections.Generic;
 using System.IO;

src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/AndroidEnumConverter.cs

@@ -1,4 +1,3 @@
-#nullable enable
 
 using System.Collections.Generic;
 

src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/AssemblyLevelElementBuilder.cs

@@ -1,4 +1,3 @@
-#nullable enable
 
 using System;
 using System.Collections.Generic;

src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/ComponentElementBuilder.cs

@@ -1,4 +1,3 @@
-#nullable enable
 
 using System;
 using System.Globalization;

src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/ManifestConstants.cs

@@ -1,4 +1,3 @@
-#nullable enable
 
 using System.Xml.Linq;
 

src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/PropertyMapper.cs

@@ -1,4 +1,3 @@
-#nullable enable
 
 using System;
 using System.Collections.Generic;

src/Xamarin.Android.Build.Tasks/Tasks/GenerateTrimmableTypeMap.cs

@@ -1,5 +1,3 @@
-#nullable enable
-
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -79,7 +77,7 @@ public override bool RunTask ()
 }
 
 ManifestConfig? manifestConfig = null;
-if (!string.IsNullOrEmpty (MergedAndroidManifestOutput) && !string.IsNullOrEmpty (PackageName)) {
+if (!MergedAndroidManifestOutput.IsNullOrEmpty () && !PackageName.IsNullOrEmpty ()) {
 manifestConfig = new ManifestConfig (
 PackageName: PackageName,
 ApplicationLabel: ApplicationLabel,