Fix: common: error query for trigger action protocol error

Fix: common: error query for trigger action protocol error

Observed issue
==============

When listing a trigger with a single non-list action the CLI reports an
error in the protocol resulting in an output with no error accounting
for the action.

 $ lttng list-triggers
  - name: trigger0
    owner uid: 1000
    condition: session rotation ongoing
      session name: test
      errors: none
   action:notify
  Error: Failed to query errors of trigger 'trigger0' (owner uid: 1000): Protocol error occurred

Cause
=====

The `action_path` associated with the query has an index count of 0 as
it should considering that the single root element action element is not
a `list` object.

Inside `lttng_action_path_create_from_payload` a payload view is
initialized with a `len` of 0 since `header->index_count` is 0 as it
should.

The payload view is then validated and is considered invalid since the
validation check for `len` > 0. The error then bubbles up.

Solution
========

Since that the payload view is considered invalid when it is equal to
zero simply handle this special case and call directly
`lttng_action_path_create` with the appropriate parameter.

Known drawbacks
=========

None.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I8f302c3aa78835342c665793908dc02f0a9dece4