fix(ftp): List empty directory

[plichard]

Solved Problem

When trying to list an empty directory, I found that the mavlink ftp implementation does not return EOF.

Solution

payload->offset == 0 should be checked as well for EOF.
This also changes the behavior to return kErrFailErrno when retrieving a dir entry fails, instead of failing silently (while removing duplicated payload->size and closedir logic).

Changelog Entry

For release notes:

Bugfix mavlink ftp not returning EOF on empty directory list.