fix(ls-metrics): prevent panic on empty directory (#1144)

Co-authored-by: VXNCXNX <VXNCXNX@users.noreply.github.com>

Author: VXNCXNX

cmd/vale/command.go

@@ -236,6 +236,14 @@ func printMetrics(args []string, _ *core.CLIFlags) error {
 		return err
 	}
 
+	// `FileExists` is true for a directory, and a directory that holds no
+	// lintable file lints to nothing. Reporting that is better than printing
+	// `{}`, which already means "a file Vale read and found no metrics in".
+	if len(linted) == 0 {
+		return core.NewE100("ls-metrics", fmt.Errorf(
+			"'%s' contains no lintable files", args[0]))
+	}
+
 	computed, _ := linted[0].ComputeMetrics()
 	return printJSON(computed)
 }

testdata/e2e/metrics.yaml

@@ -34,6 +34,21 @@ cases:
     want: |
       {}
 
+  - name: empty-directory
+    about: the argument only has to exist, and a directory does -- so one
+      whose every file is skipped (`node_modules` is ignored by default)
+      lints to nothing, and saying so beats indexing an empty result, which
+      used to panic
+    files:
+      empty/node_modules/test.md: |
+        # Title
+    args: ls-metrics empty
+    exit: 2
+    contains: |
+      E100 [ls-metrics] Runtime error
+
+      'empty' contains no lintable files
+
   - name: formula
     about: a `metric` rule reads the same counts -- the weighted total spells
       them out, so 221 means 2 H2s, 2 list items, and 1 blockquote