[FIX] t-out: does not crash when outputting null

Before this commit, doing `t-out="null"` would crash, since the
safeOutput function would assume that it is a block, thanks to the fact
that `typeof null === 'object'`.

We handle here the null value just as if it was undefined: it outputs an
empty string.

tests/compiler/__snapshots__/t_out.test.ts.snap

@@ -434,6 +434,18 @@ exports[`t-out t-out with the 0 number, in a p 1`] = `
 }"
 `;
 
+exports[`t-out top level t-out with null 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { safeOutput } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    return safeOutput(ctx['var']);
+  }
+}"
+`;
+
 exports[`t-out top level t-out with undefined 1`] = `
 "function anonymous(app, bdom, helpers
 ) {