chore: Enable nullable annotation for disassembler/diagnoser related code

[filzrev]

This PR intended to fix a part issue #2815

By enabling #nullable enable for disassembler/diagnoser related code. and fix nullable errors.

Other Changes

1. DiagnoserActionParameters.cs

• Add ProcessId getter property.

2. Arm64InstructionFormatter.cs

• Add code to handle null Instruction.

3. ClrMdArgs.cs

• Remove nullable from constructor's resultsPath argument.
• Add code to handle null Json.

4. DataContracts.cs

• Remove unused TargetFrameworkMoniker property from State.

5. MonoDisassembler.cs

• Remove nullable from input argument of Parse method.
• Remove nullable from mono argument of Disassemble method.

[timcassell]

Thanks