Fix newly introduced error in tramp-tests.el (Bug#49406)

* test/lisp/net/tramp-tests.el (tramp--test-check-files):
Filter out empty strings.  (Bug#49406)
3 jobs for master in 1 minute and 16 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Fast
failed #22816
test-fast-inotify

00:00:23

 
  Normal
failed #22817
test-lisp-inotify

00:00:13

failed #22818
test-lisp-net-inotify

00:00:52

 
Name Stage Failure
failed
test-lisp-net-inotify Normal There has been a runner system failure, please try again
Initialized empty Git repository in /builds/emacs/emacs/.git/
Created fresh repository.
From https://emba.gnu.org/emacs/emacs
* [new ref] ddf6226350934a9e9ffe436d46141c752eb440b7 -> refs/pipelines/11296
* [new branch] master -> origin/master
Checking out ddf62263 as master...

Skipping Git submodules setup
ERROR: Job failed (system failure): Error response from daemon: Conflict. The container name "/runner-2oEDrUT7-project-1-concurrent-2-build-4" is already in use by container "da1fe579c662349f5b0cbedeaf385ca8567e208eca419e08e6d3defd24c4f2dc". You have to remove (or rename) that container to be able to reuse that name. (executor_docker.go:726:0s)
failed
test-lisp-inotify Normal There has been a runner system failure, please try again
Using docker image sha256:81f5749c9058a7284e6acd8e126f2b882765a17b9ead14422b51cde1a110b85c for docker:19.03.12 ...
Running on runner-2oEDrUT7-project-1-concurrent-1 via emba.gnu.org...
$ git config --global http.proxy $HTTP_PROXY; git config --global https.proxy $HTTPS_PROXY
Fetching changes with git depth set to 50...
Reinitialized existing Git repository in /builds/emacs/emacs/.git/
Checking out ddf62263 as master...

Skipping Git submodules setup
ERROR: Job failed (system failure): Error response from daemon: Conflict. The container name "/runner-2oEDrUT7-project-1-concurrent-1-build-4" is already in use by container "f46df44a43bee1f61a6ede2eac732a4e06eaa2094659f7ea23718f093d67bcc6". You have to remove (or rename) that container to be able to reuse that name. (executor_docker.go:726:0s)
failed
test-fast-inotify Fast There has been a runner system failure, please try again
Fetching changes with git depth set to 50...
Reinitialized existing Git repository in /builds/emacs/emacs/.git/
From https://emba.gnu.org/emacs/emacs
* [new ref] ddf6226350934a9e9ffe436d46141c752eb440b7 -> refs/pipelines/11296
ed15f39..ddf6226 master -> origin/master
Checking out ddf62263 as master...

Skipping Git submodules setup
ERROR: Job failed (system failure): Error response from daemon: Conflict. The container name "/runner-2oEDrUT7-project-1-concurrent-1-build-4" is already in use by container "f46df44a43bee1f61a6ede2eac732a4e06eaa2094659f7ea23718f093d67bcc6". You have to remove (or rename) that container to be able to reuse that name. (executor_docker.go:726:0s)