Check not to try to number_format() empty string.

extension/qfq/qfq/AbstractBuildForm.php

@@ -1246,9 +1246,11 @@ abstract class AbstractBuildForm {
             $value = '';
         }
 
-        if ($formElement[FE_DECIMAL_FORMAT] !== '') {
-            $decimalScale = explode(',', $formElement[FE_DECIMAL_FORMAT])[1]; // scale: Nachkommastellen
-            $value = number_format($value, $decimalScale, '.', '');
+        if ($formElement[FE_DECIMAL_FORMAT] !== '' ) {
+            if($value!=='') { // empty string causes exception in number_format()
+                $decimalScale = explode(',', $formElement[FE_DECIMAL_FORMAT])[1]; // scale: Nachkommastellen
+                $value = number_format($value, $decimalScale, '.', '');
+            }
         }
 
         // In case the user specifies MIN & MAX with numbers, the html tag 'type' has to be 'number', to make the range check work in the browser.