Fix the termination issue of Document.Store caused by incomplete syntax errors.

[scottming]

Today, while writing an alias "as", I unexpectedly discovered that Document.Store terminated due to an unmatched error.

21:48:27.208 [error] Child :undefined of Supervisor LXical.Server.Provider.Queue.Supervisor terminated
** (exit) exited in: GenServer.call({:via, :global, {LXical.Document.Store, 322}}, {:fetch, "file:///Users/scottming/Code/dummy/lib/dummy.ex", :analysis}, 5000)
    ** (EXIT) an exception was raised:
        ** (Protocol.UndefinedError) protocol Enumerable not implemented for {:a, [_scope_id: #Reference<0.3819903562.3914334214.38740>, line: 16, column: 18], nil} of type Tuple
            (elixir 1.15.7) lib/enum.ex:1: Enumerable.impl_for!/1
            (elixir 1.15.7) lib/enum.ex:166: Enumerable.reduce/3
            (elixir 1.15.7) lib/enum.ex:1227: Enum.find_value/3
            (lx_common 0.3.0) lib/lexical/ast/analysis.ex:338: LXical.Ast.Analysis.fetch_alias_as/1
            (lx_common 0.3.0) lib/lexical/ast/analysis.ex:200: LXical.Ast.Analysis.analyze_node/2
            (lx_common 0.3.0) lib/lexical/ast/analysis.ex:71: anonymous fn/2 in LXical.Ast.Analysis.traverse/2
            (elixir 1.15.7) lib/macro.ex:688: anonymous fn/4 in Macro.do_traverse_args/4
            (stdlib 4.3.1.3) lists.erl:1462: :lists.mapfoldl_1/3
Pid: #PID<0.167.0>