Fixed bug in singlestat panel and null value handling, #2189

public/app/components/kbn.js

@@ -406,6 +406,7 @@ function($, _, moment) {
   kbn.valueFormats.velocityknot = function(value, decimals) { return kbn.toFixed(value, decimals) + ' kn'; };
 
   kbn.roundValue = function (num, decimals) {
+    if (num === null) { return null; }
     var n = Math.pow(10, decimals);
     return Math.round((n * num).toFixed(decimals))  / n;
   };

public/test/specs/kbn-format-specs.js

@@ -51,7 +51,12 @@ define([
     });
   });
 
-
+  describe('kbn roundValue', function() {
+    it('should should handle null value', function() {
+      var str = kbn.roundValue(null, 2);
+      expect(str).to.be(null);
+    });
+  });
 
   describe('calculateInterval', function() {
     it('1h 100 resultion', function() {