Don't assign nil value to named attribute mapped to absent positional attribute

[mojavelinux]

When the attrlist parser was updated to account for empty positional attributes to solve #3813, it resulted in an attribute mapped to that position being assigned a nil value. This presents a challenge for converters, which often assume that if an attribute is set, the value is non-nil.

The affect of this can be seen in the handling of the block image macro. If the positional attribute for width is empty, like in this example:

image::tux.png[Tux,]

It results in an empty width attribute in the HTML output:

<img src="tux.png" width="">

This output differs from earlier version of Asciidoctor.

The mapped attribute should only be assigned if the value is non-nil.

[mojavelinux]

It's interesting to note that the AttributeList.rekey method already works this way. It's the logic integrated with the attrlist parsing that's behaving incorrectly.