Disallow empty output directory

tests/cli-tests/basic/output_dir.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+println "+ zstd -r * --output-dir-mirror=\"\""
+zstd -r * --output-dir-mirror="" && die "Should not allow empty output dir!"
+println "+ zstd -r * --output-dir-flat=\"\""
+zstd -r * --output-dir-flat="" && die "Should not allow empty output dir!"
+exit 0

tests/cli-tests/basic/output_dir.sh.stderr.exact

@@ -0,0 +1,2 @@
+error: output dir cannot be empty string (did you mean to pass '.' instead?)
+error: output dir cannot be empty string (did you mean to pass '.' instead?)

tests/cli-tests/basic/output_dir.sh.stdout.exact

@@ -0,0 +1,2 @@
++ zstd -r * --output-dir-mirror=""
++ zstd -r * --output-dir-flat=""