Prevent crash for invalid application-properties filter

[ansd]

application-properties keys are restricted to be strings.

Prior to this commit, a function_clause error occurred if the client requested an invalid filter:

  │ *Error{Condition: amqp:internal-error, Description: Session error: function_clause
  │ [{rabbit_amqp_filtex,'-validate0/2-fun-0-',
  │                      [{{symbol,<<"subject">>},{utf8,<<"var">>}}],
  │                      [{file,"rabbit_amqp_filtex.erl"},{line,119}]},
  │  {lists,map,2,[{file,"lists.erl"},{line,2077}]},
  │  {rabbit_amqp_filtex,validate0,2,[{file,"rabbit_amqp_filtex.erl"},{line,119}]},
  │  {rabbit_amqp_filtex,validate,1,[{file,"rabbit_amqp_filtex.erl"},{line,28}]},
  │  {rabbit_amqp_session,parse_filters,2,
  │                       [{file,"rabbit_amqp_session.erl"},{line,3068}]},
  │  {rabbit_amqp_session,parse_filter,1,
  │                       [{file,"rabbit_amqp_session.erl"},{line,3014}]},
  │  {rabbit_amqp_session,'-handle_attach/2-fun-0-',21,
  │                       [{file,"rabbit_amqp_session.erl"},{line,1371}]},
  │
  {rabbit_misc,with_exit_handler,2,[{file,"rabbit_misc.erl"},{line,465}]}],
  Info: map[]}

After this commit, the filter won't actually take effect without a crash occurring.

Supersedes #12520