update multicurrencies functionality on global level

vendor/wpcfto/metaboxes/metabox.php

@@ -162,11 +162,13 @@ class STM_Metaboxes {
 	}
 
 	public function wpcfto_save_number( $value ) {
-		$value = floatval( $value );
-		if ( 0 === $value ) {
+		// Preserve zero as a valid value; do not coerce to empty string.
+		// This aligns server-side with client-side required validation.
+		if ($value === '' || $value === null) {
 			return '';
 		}
-
+		// Allow string "0", numeric 0, or any float.
+		$value = is_numeric($value) ? $value + 0 : $value;
 		return $value;
 	}