refactor(pure): remove duplicate regex and dead empty-string guard

tests/pure.test.js

@@ -30,4 +30,5 @@ test('parseTarget: rejects invalid input', () => {
   assert.equal(parseTarget(undefined), null);
   assert.equal(parseTarget('25M.5'), null);
   assert.equal(parseTarget('1.5.5M'), null);
+  assert.equal(parseTarget('0'), null);
 });