"Show all posts" option in category dropdown list link doesnt work

I have a dropdown list showing categories for my blog. the “Show All Posts” option shows up, but when selected doesnt go anywhere, the link is just broken. When I select a category the links work fine, only the “Show all posts” link is broken. anyone have any idea how to fix?

<span class="custom-dropdown custom-dropdown--white">
<?php $args = array(
	'show_option_none' => 'Select category',
        'show_option_all' => 'All Posts'
); ?>
<?php wp_dropdown_categories($args); ?>
</span>
<script type="text/javascript">
	<!--
	var dropdown = document.getElementById("cat");
	function onCatChange() {
		if ( dropdown.options[dropdown.selectedIndex].value > 0 ) {
			location.href = "<?php echo esc_url( home_url( '/' ) );
?>?cat="+dropdown.options[dropdown.selectedIndex].value;
		}
	}
	dropdown.onchange = onCatChange;
	-->
</script>

I tried this:

<script type="text/javascript">
	<!--
	var dropdown = document.getElementById("cat");
	function onCatChange() {
		if ( dropdown.options[dropdown.selectedIndex].value > 0 ) {
			location.href = "<?php echo esc_url( home_url( '/' ) ); ?>?cat="+dropdown.options[dropdown.selectedIndex].value;
		} elseif( dropdown.options[dropdown.selectedIndex].value == 0 ){
			location.href = "<?php echo esc_url( home_url( '/' ) ); ?>blog";
		}
	}
	dropdown.onchange = onCatChange;
	-->
</script>

but that code breaks all the category links