Let's keep null around. At least one example (*cough* openlayers homepage *cough*) uses it, although that should obviously have been documented in a test.

git-svn-id: http://svn.openlayers.org/trunk/openlayers@814 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Map.js

@@ -195,7 +195,7 @@ OpenLayers.Map.prototype = {
         // if maxResolution is specified as "auto", calculate it 
         //  based on the maxExtent and the viewSize
         //
-        if (this.maxResolution == "auto") {
+        if (this.maxResolution == "auto" || this.maxResolution == null) {
             var maxExtent = this.getMaxExtent();
             var viewSize = this.getSize();
             this.maxResolution = Math.max(maxExtent.getWidth()  / viewSize.w,