Modifier doesn't check if modifier is empty!

[VishalKumarSahu]

bem-twig-extension/bem.function.php

Line 19 in c772316

foreach ($modifiers as $modifier) {

If we pass an array of modifer like
bem(base_element, ['mod1', 'mod2', ''])
returns
base_element--mod1 base_element--mod2 base_element--
where last part is undesirable, since inside twig some variable may be empty. So there should be some check for if the value is there or it is empty.

[jeff-cardwell]

Whoops. I didn't see this issue before I created PR #15. It should fix this.