nf-graph: better behavior when setting yMin, yMax to null

Previously, setting `yMin` or `yMax` to `undefined` caused

> Error: Invalid value for <g> attribute transform="translate(520, NaN)"

There was no way to set `yMin` or `yMax` to "something if it's defined,
or default behavior otherwise". This now treats the following as
equivalent:

```hbs
{{!-- no yMax specified: --}}
{{nf-graph}}

{{!-- yMax set to undefined: --}}
{{nf-graph yMax=somePropertyThatIsUndefined}}
```

app/components/nf-graph.js

@@ -131,7 +131,7 @@ var minProperty = function(axis, defaultTickCount){
       var mode = this.get(_MinMode_);
       var ext;
 
-      if(arguments.length > 1) {
+      if(value != null) {
         this[__Min_] = value;
       } else {
         var change = function(val) {
@@ -183,7 +183,7 @@ var maxProperty = function(axis, defaultTickCount) {
       var mode = this.get(_MaxMode_);
       var ext;
 
-      if(arguments.length > 1) {
+      if(value != null) {
         this[__Max_] = value;
       } else {
         var change = function(val) {