Better test for not printing NULL

Author: aitap

inst/tests/tests.Rraw

@@ -11653,8 +11653,7 @@ test(1765.4, {warning("foobar1"); warning("foobar2"); warning("FOO"); 4L}, 4L, i
 test(1765.5, {warning("foobar1"); warning("foobar2"); warning("FOO"); 4L}, 4L, ignore.warning="2", warning=c("foobar1","FOO"))
 
 # print(null.data.table()) should not output NULL as well, #1852
-# the ^, $ specifiers don't match embedded newlines (if any) so match will fail if NULL is also printed
-test(1766, data.table(NULL), output = "^Null data\\.table \\(0 rows and 0 cols\\)$")
+test(1766, data.table(NULL), notOutput = "NULL$")
 
 # Bug on subset of 1-row data.table when expr returns a named logical vector #2152
 options(datatable.auto.index=FALSE)