b2/: insert a virtual empty hierarchy if test input starts with other file

Also updated the regression tests that explicitly provided an empty
hierarchy.

b2/test/Common

@@ -13,13 +13,17 @@
 
 run_boom()
 {
-	args="-N h <(sed -n '/^!-/q;p' _in)"
+	if [ "`sed -n '/^!-/p;q' _in`" ]; then
+		args="<(echo ';')"
+	else
+		args="-N h <(sed -n '/^!-/q;p' _in)"
+	fi
 	for n in c x p i s b X; do
 		for m in "" 1 2 3; do
 			if grep ^!-$n$m\$ _in >/dev/null; then
 				args="$args -N $n$m"
 				args="$args -$n \
-<(sed -n '1,/^!-$n$m/d;/^!-/q;p' _in)"
+<(sed -n '0,/^!-$n$m/d;/^!-/q;p' _in)"
 			fi
 		done
 	done