Truthiness check in case of SafeString

[vecmezoni]

Macros can return SafeString object with empty val. If you'll make truthiness check of this object it would be true, but empty string would be false.

Any suggestions on fixing this issue? I was thinking about adding special truthiness check function, but then we should check everything in code with this function.

BTW jinja works fine in this case.
Markup('') is False

[carljm]

This is somewhat related to #267. I think both would be fixed in a similar way, by introducing a "truthiness check" function and using it rather than simple JS truthiness check.