Fix: Fixed a NullReferenceException if printing null text

[NicusorN5]

Fixes this exception if using DebugText.Print(null, ...) by adding a plain null check inside the Print function.

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=Stride.Graphics
  StackTrace:
   at Stride.Graphics.FastTextRenderer.DrawString(GraphicsContext graphicsContext, String text, Int32 x, Int32 y)

Related Issue

None, this is just a trivial bug fix. I didn't decide to create a issue for this.

Types of changes

• N/A Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• N/A New feature (non-breaking change which adds functionality)
• N/A Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• N/A My change requires a change to the documentation.
• N/A I have added tests to cover my changes.
• N/A All new and existing tests passed.
• I have built and run the editor to try this change out.
  (by creating a new project, and using this script containing a single DebugText.Print call inside the Update method).

[Kryptos-FR]

LGTM. Thanks