readline_read/write_history's parameter is optional

When passing `NULL` to ` write_history` or `read_history` `~/.history` will be used.

Also stop returning false on success and true on failure...

Test Plan:
test/run `test/zend/good/ext/readline/`
`test/slow/ext_readline/` 

I am not sure if I can properly test for this without being flaky, so just a test if passing no arguments work.