cli-plugins/manager: add test for empty / non-existing plugin dirs

Verify that listPluginCandidates returns an empty result if nothing was
found.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

cli-plugins/manager/manager_test.go

@@ -1,6 +1,7 @@
 package manager
 
 import (
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -81,6 +82,12 @@ func TestListPluginCandidates(t *testing.T) {
 	assert.DeepEqual(t, candidates, exp)
 }
 
+func TestListPluginCandidatesEmpty(t *testing.T) {
+	tmpDir := t.TempDir()
+	candidates := listPluginCandidates([]string{tmpDir, filepath.Join(tmpDir, "no-such-dir")})
+	assert.Assert(t, len(candidates) == 0)
+}
+
 // Regression test for https://github.com/docker/cli/issues/5643.
 // Check that inaccessible directories that come before accessible ones are ignored
 // and do not prevent the latter from being processed.