Random "Empty first server packet" errors when copying files

When copying directories, cf-agent regularly stops copy with errors like:

<pre>
error: Empty first server packet when listing directory '/var/rudder/share/c45e0e9f-5d9b-4685-b26c-35a3dcf35954/rules/cfengine-community/clockConfiguration/3.0'!
</pre>

It seems to happen since 3.7.0 with https://github.com/cfengine/core/commit/766f991dc61e589a1d8b98274d946df28e3003ef, which adds an empty string test in RemoteDirList, but checks the first char before calling DecryptString, which leads to random errors.