Happy New Year and good afternoon,

Since some l3kernel update in December I am getting kernel/invalid
-variant warnings, as in  

* LaTeX warning: "kernel/invalid-variant"
* Variant form 'cF' invalid for base form '\tl_if_blank:nF'.

It is issued for the line of code


that used to work without warning before. Which is the correct method for
generating variants of conditionals?