Add wildcard behavior for null endpoints in rangeQuery.

I believe the '*' range feature is a pretty common usecase of range
queries.  Right now, passing null to this function generates an invalid
SOLR phrase like 'field:[5 TO ]'.  Users can of course pass '*' to the
helper, but supporting null makes this common usecase easier to handle.

For example,

```php
$minValue = $request->get('min'); // returns null if min not specified
$maxValue = $request->get('max'); // returns null if max not specified
$range = $helper->rangeQuery('value', $minValue, $maxValue);
```