Treat null values as falsey in inverted sections

[TheInnerLight]

This PR changes the handle of the null values.

If some supplied value takes a null value and we have two cases:

Case 1:

{{#value}}
    Something.
{{/value}}

Case 2:

{{^value}}
    Something.
{{/value}}

Both cases currently result in nothing being rendered.

This PR changes this such that, in Case 2, "Something" is rendered. This change ensures conformance with the mustache spec which clarifies that for null values in inverted sections, the text should be rendered (https://github.com/mustache/spec/blob/master/specs/inverted.yml#L49) as discussed in this issue.