add a reusable empty function

[tschaub]

For places where we create an anonymous empty function, we should be able to reuse one defined elsewhere.

[tschaub]

void

[tschaub]

Tests pass in IE6 and FF3 (date parsing needs fixed in Chrome - unrelated).

[ahocevar]

Looks good. Please commit. The only comment I have is that you could change the line in the test to follow the t.eq(got, expected, message) signature:

t.eq(OpenLayers.Function.Void(), undefined, "returns undefined");

[fredj]

r11172 and r11174