fixes to examples of template expansion of undefined (null) values

RFC6570 section 3.2.1 says:
"A variable that is undefined (Section 2.3) has no value and is ignored by the
expansion process.  If all of the variables in an expression are undefined,
then the expression's expansion is the empty string."

..therefore the provided examples for the "matrix" and "label" styles are
incorrect -- the full expansion is the empty string, with no leader/prefix.