prevent exception when using leftnocase,,likenocase etc

Fixes following issue:
// Note: last city added has null value for name
var cities = TAFFY([{name:"New York",state:"WA"},{name:"Las Vegas",state:"NV"},{name:null,state:"MA"}]);

// Query for cities whose name starts with N or n
cities({name:{leftnocase:'n'}}).get();

TypeError: Cannot read property 'toLowerCase' of null