#68 Tag return values that equate to false aren't handled

[Travis Miller]

If a tag returns a value that casts to false in a boolean context (such as the string "0"), template_exec_tag() in lib/templates.php returns the tag's altoutput instead.

I'm working on a module to import and display AccuWeather forecast data, and there are cases where tags need to return "0" (windspeed, temperature, etc.). As a workaround, I've appended a single character of whitespace at the end of the tag's return value, but I can imagine cases where this would be problematic.

template_exec_tag() should only use altoutput if ( $output === '' || $output === false ).