[clang-format] Rename CSharpNullConditionalSq and add missing test

Summary:
Rename CSharpNullConditionalSq to CSharpNullConditionalLSquare.

Add test for spaces inside [] with C# Null conditionals.

Address comments missed from https://reviews.llvm.org/D75368.

Reviewers: krasimir

Reviewed By: krasimir

Subscribers: cfe-commits

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D75456