Fixed JoinSqlBuilder for SqlLite

With SqlLite provider the FieldDefinition.Alias doesn't default to
FieldDefinition.Name like with SqlServer or Firebird.

Added a check for null or empty to FieldDefinition.Alias before use.

var fieldName = String.IsNullOrEmpty(fieldDef.Alias) ? fieldDef.Name :
fieldDef.Alias;
Merge pull request #329 from madaleno/JoinSqlBuilder

Fixed JoinSqlBuilder for SqlLite