Editing cells with null values

[GoogleCodeExporter]

What steps will reproduce the problem?
1. click "edit" on a cell with a null value
2. change the value and apply to all identical cells

What is the expected output? What do you see instead?
One would expect all cells with null values to update. Instead no cells are 
changed, including the cell from which the edit was made.

What version of Google Refine are you using?
Version 2.0 [r1836]

What operating system and browser are you using?
Windows XP SP3, Chrome 9.0.597.84

Is this problem specific to the type of browser you're using or it happens
in all the browsers you tried?
Same issue using FF 3.6.13

Please provide any additional information below.
Dealing with nulls is generally problematic. Using any of the scripting tools 
to access null values produces errors. And it would be helpful to be able to 
differentiate nulls from blanks in the actual grid view of the data.

Original issue reported on code.google.com by benjamin...@gmail.com on 8 Feb 2011 at 7:42

[GoogleCodeExporter]

Actually, I think this issue or question could go even higher level.  Would it 
be useful for the user to be presented with an option during import to 
automatically promote a null to a blank ?(typically a null = NOTHING found 
between a pair of commas in a CSV file or a pair of \t tabs in a TSV file)  
`Earth,,New York City  vs Earth \t\t New York City`

Original comment by thadguidry on 8 Feb 2011 at 8:50

[GoogleCodeExporter]

I think an "import empty values as nulls vs. blanks" would be useful, as would 
a means to do this after import. As of now I can't transform any null cells to 
blanks, and have resorted to using exception handling to merge columns together.

In any case, it looks like issue 252 is related. When a cell is null, accessing 
the value attribute throws an error. They used GREL, I encountered the same 
through Jython.

Original comment by benjamin...@gmail.com on 8 Feb 2011 at 9:55

[GoogleCodeExporter]

I'll be sure to add that option to the new importer UI.

Original comment by dfhu...@gmail.com on 10 Feb 2011 at 1:14