[dcl.attr.contract] Clarify that assertions may apply to a null statement

Author: jensmaurer

source/declarations.tex

@@ -4453,9 +4453,10 @@
 \end{example}
 
 \pnum
+An assertion may be applied to a null statement\iref{stmt.expr}.
 An assertion is checked by evaluating its predicate
-as part of the evaluation of the null statement\iref{stmt.expr}
-it applies to. 
+as part of the evaluation of the null statement
+it applies to.
 
 \pnum
 The only side effects of a predicate