Whitespace alone should be ignored in accName algorithms

[scottaohara]

We need to make it clear in the specification that attributes that contribute to the accessible name of elements should be ignored if, when white space is trimmed, they return the empty string.

e.g. alt=" " should be treated the same as alt="".

[pkra]

Related: w3c/accname#55, whitespace is defined inconsistently across specs.

[scottaohara]

I'm closing this issue in favor of w3c/accname#55

handling whitespace in accNames should be defined consistently over there, not with separate rules here.