When running a custom action without any files selected, the substitution parameters (%F, %f etc.) act as if the directory where the action was started is selected. That is, if I run the action in directory /a/b/c without any files selected, %f and %F expand to '/a/b/c', %d and %D expand to '/a/b', %n and %N expand to 'c'. If no files are selected, I would expect all those to be empty strings.
ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: thunar 1.6.12-1
ProcVersionSignature: Ubuntu 4.13.0-38.43-generic 4.13.16
Uname: Linux 4.13.0-38-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.7-0ubuntu3.7
Architecture: amd64
Date: Thu Apr 5 19:00:32 2018
ExecutablePath: /usr/bin/thunar
InstallationDate: Installed on 2017-02-04 (425 days ago)
InstallationMedia: Xubuntu 16.10 "Yakkety Yak" - Release amd64 (20161012.2)
ProcEnviron:
XDG_RUNTIME_DIR=<set>
SHELL=/bin/zsh
PATH=(custom, user)
LANG=C.UTF-8
LD_LIBRARY_PATH=<set>
SourcePackage: thunar
UpgradeStatus: Upgraded to artful on 2017-11-01 (154 days ago)
My suggested changes for plugins/
"the path to the first selected file" -> "the path to the first selected file or directory"
"the paths to all selected files" -> "the paths to all selected files and directories"
The remaining labels could be adjusted also, but those two changes may already be enough to clarify that the selection applies to files and directories.