Fixed the type returned when get_stock_quantity() is empty

Since expect return an integer value, we can't return an empty string.
This can generate some kinds of errors like:

 '' >= 1 returns false
 '' >= -1 returns true

This can be a problem comparing stock quantity like we do in
WC_Product::has_enough_stock()

But using null we have:

 null >= 1 returns false
 null >= -1 returns false

cc @mikejolley

ref #9598

includes/abstracts/abstract-wc-product.php

@@ -196,7 +196,7 @@ class WC_Product {
 	 * @return int
 	 */
 	public function get_stock_quantity() {
-		return apply_filters( 'woocommerce_get_stock_quantity', $this->managing_stock() ? wc_stock_amount( $this->stock ) : '', $this );
+		return apply_filters( 'woocommerce_get_stock_quantity', $this->managing_stock() ? wc_stock_amount( $this->stock ) : null, $this );
 	}
 
 	/**