parse.y/directives: Reject directives as statements

Avoid the issue outlined in #2289 where the @if or @else is taken as the
statement of an `if`, `for` or `while` by rejecting such constructs.

Effectively this means the following scripts are now rejected:

    # Print's "cond true" with Zeek 5.0 even though the `if ( F )`
    # should be in effect.

    if ( F )
        @if ( T )
            print "cond true";
        @else
            print "cond false";
        @endif

or

    # Print's "hello" once with Zeek 5.0
    local v = vector( 1, 2, 3 );

    for ( i in v )
        @if ( T )
        print("hello")
        @endif

To make above work as intended, additional braces can be used.

    if ( T )
        {
    @if ( cond )
            print "cond true";
    @else
            print "cond false";
    @endif
        }

    for ( i in v )
        {
    @if ( T )
        print("hello")
    @endif
        }

src/StmtBase.h

@@ -34,6 +34,7 @@ class ExprStmt;
 class ForStmt;
 class IfStmt;
 class InitStmt;
+class NullStmt;
 class PrintStmt;
 class ReturnStmt;
 class StmtList;
@@ -92,6 +93,7 @@ public:
 	const WhileStmt* AsWhileStmt() const;
 	const WhenStmt* AsWhenStmt() const;
 	const SwitchStmt* AsSwitchStmt() const;
+	const NullStmt* AsNullStmt() const;
 
 	void RegisterAccess() const
 		{