Fix: event field value: assertion fails on empty string

Fix: event field value: assertion fails on empty string

When converting msgpack objects to their event_field_value equivalent,
the following assertion fails: LTTNG_ASSERT(val);

 #4  0x00007f1f65349486 in __assert_fail () from /usr/lib/libc.so.6
 #5  0x00007f1f65584da7 in lttng_event_field_value_string_create_with_size (val=0x0, size=0) at event-field-value.cpp:186
 #6  0x00007f1f65576a1a in event_field_value_from_obj (obj=0x557f597ccdb8, field_val=0x7ffcc9675dd0)
     at conditions/event-rule-matches.cpp:1120
 #7  0x00007f1f65577176 in event_field_value_from_capture_payload (condition=0x557f597c8520,
     capture_payload=0x557f597c825b "\221\240", capture_payload_size=2) at conditions/event-rule-matches.cpp:1340
 #8  0x00007f1f655772ea in lttng_evaluation_event_rule_matches_create (condition=0x557f597c8520,
     capture_payload=0x557f597c825b "\221\240", capture_payload_size=2, decode_capture_payload=true)
     at conditions/event-rule-matches.cpp:1398
 #9  0x00007f1f655765fc in lttng_evaluation_event_rule_matches_create_from_payload (condition=0x557f597c8520,
     view=0x7ffcc9675ff0, _evaluation=0x7ffcc9676080) at conditions/event-rule-matches.cpp:990
 #10 0x00007f1f6557f273 in lttng_evaluation_create_from_payload (condition=0x557f597c8520, src_view=0x7ffcc9676100,
     evaluation=0x7ffcc9676080) at evaluation.cpp:120
 #11 0x00007f1f6559ba36 in lttng_notification_create_from_payload (src_view=0x7ffcc9676190, notification=0x7ffcc9676180)
     at notification.cpp:123
 #12 0x00007f1f65552577 in create_notification_from_current_message (channel=0x557f597c8ee0) at channel.cpp:124
 #13 0x00007f1f6555298c in lttng_notification_channel_get_next_notification (channel=0x557f597c8ee0, _notification=0x7ffcc9676280)
     at channel.cpp:292

The msgpack API represents string as p-style while the implementation of
event_field_value relies on null-terminated strings. When an empty
string is captured by a tracer, it is decoded as a msgpack_object with
`str = {size = 0, ptr = 0x0}`.

lttng_event_field_value_string_create_with_size does not require a
null-terminated string since it also receives the length. Hence, this
fix causes lttng_event_field_value_string_create_with_size to accept
null strings when their length is zero. A copy of an empty string is
made to accomodate the null-termination convention used by the rest of
that API.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I7c3a839dbbeeb95a1b3bf6ddc3205a2f6b4538e3