webform_block table is populated incorrectly

Great module, extremely helpful.

A couple of things that i've noticed though which i believe i've corrected in the attached patch.

1) in the current releases of the module, it seems that there is not adequate checking to see if we should be doing any sort of operation, such as inserting a record into the 'webform_block' table. In hook_nodeapi, i've added a check to ensure that we're working with a webform node type. Currently the nid of every node that gets inserted into the site gets entered into this table regardless of content type.

2) i've added some minor attribute checks to eliminate warning messages that were showing up on the site i'm working on. These checks don't change the logic, or the functionality, just eliminate the warning messages.