Enable nullable reference types

[RyanLua]

Description

Since Fluent Auto Clicker's codebase is pretty small, it is recommended to add nullable reference types since it would require minimal work and allow improved diagnostics.

To achieve this, you can add the below Nullable option to the FluentAutoClicker.csproj and then address warnings.

<Nullable>enable</Nullable>

Supporting information

https://learn.microsoft.com/en-us/dotnet/csharp/nullable-migration-strategies
https://learn.microsoft.com/en-us/dotnet/csharp/nullable-references
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/language#nullable