NSBrowser small fix

[Ludovic Marcotte]

Hi,

In NSBrowser: -_getTitleOfColumn we have:

...
      if ([c isLeaf])
        {
          return @"";
        }
      else
        { 
          return [c stringValue];
        }

and it should be:

      if ([c isLeaf] || [c stringValue] == nil)
        {
          return @"";
        }
      else
        { 
          return [c stringValue];
        }

otherwise, we will have an assertion failed (since we'll call
NSCell: -setStringValue: nil) when NSBrowser objects have more than one
empty column.

It fixes the crash we have in NSOpenPanel and NSSavePanel when you
maximise them and they both have empty columns.

Ludo

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