Fixing null and false attributes

[weaverryan]

Hi!

Suppose this config:

webpack_encore:
    script_attributes:
        defer: false
        async: null

This PR changes what this outputs:

<!-- before -->
<script src="..." defer="" async=""></script>

<!-- after -->
<script src="..." async></script>

This was an oversight when I built the feature.

With this PR, false correctly means "do not show this attribute". null is a bit trickier: I chose to make this mean "show the attribute, but with value (e.g. <script defer>). Any other values are rendered normally.

Cheers!

[maxhelias]

It fix this issue : #103