Fix: src.ctf.fs: metadata-info: sanitize `path` param

Fix: src.ctf.fs: metadata-info: sanitize `path` param

Issue
=====
Omitting to pass the mandatory `path` parameter to the `metadata-info`
query results in a BT_ASSERT() failure when calling
bt_value_get_string(), or a NULL pointer dereference in non-DEV_MODE.

Similarly, setting the `path` to a non-string value would also result in
a BT_ASSERT() failure, or a invalid memory access in non-DEV_MODE

Solution
========
Confirm that the `path` parameter is present and is of the string type and
print error messages accordingly it's not.

Known drawbacks
===============
None.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I8182de2797c375262d77a4a8961bedb4c5ef9578
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1081
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>