Filters empty text

Until #1821178: Performance tune text_field_load() gets put into core, I'd suggest we do something locally. I noticed something strange with really short entries in a node's 'value'. So short that there is no 'summary'. It still passed the text value of the body to htmlpurifer. This is unnecessary, since the text is an empty string. I'd like to suggest we exit _htmlpurifier_process_text early.