[diagnostics] rel.location can be null

[kLabz]

relatedInformation's location can be null:

• https://github.com/HaxeFoundation/haxe/blob/ddef638/src/context/display/diagnosticsPrinter.ml#L223
• https://github.com/HaxeFoundation/haxe/blob/c850b65/src/core/json/genjson.ml#L117-L121

Should avoid the following:

Though I didn't have a repro for this atm.

Note: should probably update https://github.com/vshaxe/language-server-protocol-haxe/blob/master/src/languageServerProtocol/Types.hx#L229 too?

[kLabz]

There's actually quite a bit of range and location that can end up being null that are currently not handled.. setting those as optional opened a nice rabbit hole, will see if I can make it work in a follow-up PR (well, likely several PRs spanning Haxe std, language-server-protocol-haxe, Haxe LSP...)