fix: do not crash when Expert.Protocol.Conversions.to_elixir/2 receives nil as document

[eksperimental]

I was experiencing this error on VS Code.

[info]   Message: ** (FunctionClauseError) no function clause matching in XPExpert.Protocol.Conversions.to_elixir/2
    (xp_expert 0.1.0-399b2a0) lib/expert/protocol/conversions.ex:22: XPExpert.Protocol.Conversions.to_elixir(%XPGenLSP.Structures.Range{end: %XPGenLSP.Structures.Position{character: 0, line: 0}, start: %XPGenLSP.Structures.Position{character: 0, line: 0}}, nil)
    (xp_forge 0.1.0-399b2a0) lib/forge/protocol/convertible.ex:52: anonymous fn/4 in XPForge.Protocol.Convertible.Helpers.apply/3
    (elixir 1.17.3) lib/enum.ex:4858: Enumerable.List.reduce/3
    (elixir 1.17.3) lib/enum.ex:2585: Enum.reduce_while/3
    (xp_forge 0.1.0-399b2a0) lib/forge/protocol/convertible.ex:182: XPForge.Protocol.Convertible.Any.to_native/2
    (xp_expert 0.1.0-399b2a0) lib/expert/protocol/convert.ex:26: XPExpert.Protocol.Convert.to_native/1
    (xp_expert 0.1.0-399b2a0) lib/expert.ex:119: XPExpert.handle_request/2
    (xp_gen_lsp 0.11.2) lib/gen_lsp.ex:369: anonymous fn/2 in XPGenLSP.loop/3

  Code: -32603