[Metal] Multiple APIs missing [NullAllowed]

### Steps to Reproduce

1. Create or load any boilerplate Metal example project.
2. Attempt to call SetFragmentTexture(null, 0) on any IMTLRenderCommandEncoder.

### Expected Behavior

The call is accepted.

### Actual Behavior

Unhandled Exception:

System.ArgumentNullException: Value cannot be null.
Parameter name: texture

### Environment

```
Microsoft Visual Studio Community 2019
Version 16.0.1
VisualStudio.16.Release/16.0.1+28803.156
Microsoft .NET Framework
Version 4.7.03190

Installed Version: Community

Visual C++ 2019 00435-60000-00000-AA963
Microsoft Visual C++ 2019

Application Insights Tools for Visual Studio Package 9.0.20307.1
Application Insights Tools for Visual Studio

ASP.NET and Web Tools 2019 16.0.12311.10635
ASP.NET and Web Tools 2019

ASP.NET Web Frameworks and Tools 2019 16.0.12311.10635
For additional information, visit https://www.asp.net/

Azure App Service Tools v3.0.0 16.0.12311.10635
Azure App Service Tools v3.0.0

Azure Functions and Web Jobs Tools 16.0.12311.10635
Azure Functions and Web Jobs Tools

C# Tools 3.0.0-beta4-19170-01+1deafee3682da88bf07d1c18521a99f47446cee8
C# components used in the IDE. Depending on your project type and settings, a different version of the compiler may be used.

Common Azure Tools 1.10
Provides common services for use by Azure Mobile Services and Microsoft Azure Tools.

Extensibility Message Bus 1.1.77 (master@24013d5)
Provides common messaging-based MEF services for loosely coupled Visual Studio extension components communication and integration.

Microsoft Azure Tools 2.9
Microsoft Azure Tools for Microsoft Visual Studio 0x10 - v2.9.0.0

Microsoft Continuous Delivery Tools for Visual Studio 0.4
Simplifying the configuration of Azure DevOps pipelines from within the Visual Studio IDE.

Microsoft JVM Debugger 1.0
Provides support for connecting the Visual Studio debugger to JDWP compatible Java Virtual Machines

Microsoft Library Manager 1.0
Install client-side libraries easily to any web project

Microsoft MI-Based Debugger 1.0
Provides support for connecting Visual Studio to MI compatible debuggers

Microsoft Visual C++ Wizards 1.0
Microsoft Visual C++ Wizards

Microsoft Visual Studio Tools for Containers 1.1
Develop, run, validate your ASP.NET Core applications in the target environment. F5 your application directly into a container with debugging, or CTRL + F5 to edit & refresh your app without having to rebuild the container.

Microsoft Visual Studio VC Package 1.0
Microsoft Visual Studio VC Package

Mono Debugging for Visual Studio 16.0.300 (573eda3)
Support for debugging Mono processes with Visual Studio.

NuGet Package Manager 5.0.0
NuGet Package Manager in Visual Studio. For more information about NuGet, visit http://docs.nuget.org/.

ProjectServicesPackage Extension 1.0
ProjectServicesPackage Visual Studio Extension Detailed Info

ResourcePackage Extension 1.0
ResourcePackage Visual Studio Extension Detailed Info

ResourcePackage Extension 1.0
ResourcePackage Visual Studio Extension Detailed Info

SQL Server Data Tools 16.0.61903.25110
Microsoft SQL Server Data Tools

Test Adapter for Boost.Test 1.0
Enables Visual Studio's testing tools with unit tests written for Boost.Test. The use terms and Third Party Notices are available in the extension installation directory.

Test Adapter for Google Test 1.0
Enables Visual Studio's testing tools with unit tests written for Google Test. The use terms and Third Party Notices are available in the extension installation directory.

TypeScript Tools 16.0.10306.2001
TypeScript Tools for Microsoft Visual Studio

Visual Basic Tools 3.0.0-beta4-19170-01+1deafee3682da88bf07d1c18521a99f47446cee8
Visual Basic components used in the IDE. Depending on your project type and settings, a different version of the compiler may be used.

Visual C++ for Cross Platform Mobile Development (Android) 15.0.28625.91
Visual C++ for Cross Platform Mobile Development (Android)

Visual F# Tools 10.4 for F# 4.6 16.0.0.0. Commit Hash: 809f41207b74a1356208f11b8fa3a15233734050.
Microsoft Visual F# Tools 10.4 for F# 4.6

Visual Studio Code Debug Adapter Host Package 1.0
Interop layer for hosting Visual Studio Code debug adapters in Visual Studio

Visual Studio Tools for CMake 1.0
Visual Studio Tools for CMake

Visual Studio Tools for CMake 1.0
Visual Studio Tools for CMake

Visual Studio Tools for Containers 1.0
Visual Studio Tools for Containers

VisualStudio.Mac 1.0
Mac Extension for Visual Studio

Xamarin 16.0.0.515 (d16-0@a3502d929)
Visual Studio extension to enable development for Xamarin.iOS and Xamarin.Android.

Xamarin Designer 4.17.4.418 (remotes/origin/d16-0@3d086e814)
Visual Studio extension to enable Xamarin Designer tools in Visual Studio.

Xamarin Templates 16.1.47 (31f664d)
Templates for building iOS, Android, and Windows apps with Xamarin and Xamarin.Forms.

Xamarin.Android SDK 9.2.0.5 (HEAD/292d27fa)
Xamarin.Android Reference Assemblies and MSBuild support.
Mono: mono/mono/2018-08-rc@5ad371dab1b
Java.Interop: xamarin/java.interop/d16-0@c987483
LibZipSharp: grendello/LibZipSharp/master@44de300
LibZip: nih-at/libzip/rel-1-5-1@b95cf3f
MXE: xamarin/mxe/xamarin@b9cbb535
ProGuard: xamarin/proguard/master@905836d
SQLite: xamarin/sqlite/3.26.0@325e91a
Xamarin.Android Tools: xamarin/xamarin-android-tools/d16-0@0a7edd6

Xamarin.iOS and Xamarin.Mac SDK 12.8.0.0 (0aa8452)
Xamarin.iOS and Xamarin.Mac Reference Assemblies and MSBuild support.
```