Fix exception with AmountFormatter if Si prefix were enabled and no unit symbol were set.

Also this case is now prevented by Validator (as it is not very useful). Fixes issue #171.

src/Services/AmountFormatter.php

@@ -131,7 +131,8 @@ class AmountFormatter
                     /** @var MeasurementUnit $unit */
                     $unit = $options['measurement_unit'];
 
-                    return $unit->getUnit();
+                    //When no unit is set, return empty string so that this can be formatted properly
+                    return $unit->getUnit() ?? '';
                 }
 
                 return '';