Handle null value for bindonce value.

Here's how I'm using BindOnce.

        <!-- Planning Price may be null. -->
        <span bindonce="row.Planning_Price__c">
            <span bo-text="row['Planning_Price__c'] | noFractionCurrency"
            /> <!-- This filter will output $0 if input is null. -->
        </span>

When Planning Price is null, the `checkBindonce` function in BindOnce is throwing an exception, as it's trying to find a `then` function on `null`.

To solve this, we can consider `null` a special case, as a guard against odd arguments. I'm open to alternative implementations, but I consider this pretty readable.

I used this UglifyJS site to create the `bindonce.min.js` file - http://marijnhaverbeke.nl/uglifyjs 

Thoughts?