Add a test for tl_if_blank

This simply makes sure that tl_if_blank:n does not
first turn its argument to a string, as some non-blank
token lists have blank string representations.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5069 de43f980-851b-0410-b2f7-c40aca1f87e0
Fix some code explanations in l3tl


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5070 de43f980-851b-0410-b2f7-c40aca1f87e0