Fixed various issues with Table plugin API not doing null checks.

The Table plugin API fails in various situations when calling execCommand without a valid table/cell selection. This is easily reproducible in the TinyMCE demo, ex: 
1)Open the TinyMCE demo, don't select a table
2a)tinymce.activeEditor.execCommand('mceTableRowProps')
2b)tinymce.activeEditor.execCommand('mceTableDeleteRow')
etc.
I won't list all situations where the plugin crashes here. 

I also realize that I probably should not be making these API calls without a valid table/cell selection, but at the same time if TinyMCE is going to support an API it should except to handle fail conditions.