(alx-0014r6-H) Refactor syntax of null directives

Signed-off-by: Alejandro Colomar <[email protected]>

Author: alejandro-colomar

source/preprocessor.tex

@@ -55,7 +55,7 @@
     line-directive\br
     diagnostic-directive\br
     pragma-directive\br
-    \terminal{\# }new-line
+    null-directive
 \end{bnf}
 
 \begin{bnf}
@@ -2201,6 +2201,11 @@
 \rSec1[cpp.null]{Null directive}%
 \indextext{preprocessing directive!null}
 
+\begin{bnf}
+\nontermdef{null-directive}\br
+    \terminal{\# }new-line
+\end{bnf}
+
 \pnum
 A preprocessing directive of the form
 \begin{ncsimplebnf}