elm_interface_scrollable: prevent calling APIs with NULL edje object

elm_interface_scrollable: prevent calling APIs with NULL edje object

Summary:
Is is unnecessary to call edje_object_ functions if `edje_obj` is NULL.

Also, It prevents printing 6 eo warnings
```
WRN<30343>:eo ../src/lib/eo/eo.c:644 _efl_object_call_resolve() NULL passed to function efl_layout_signal_emit().
```

Test Plan:
`EINA_LOG_LEVELS=eo:2 elementary_test`
`EINA_LOG_LEVELS=eo:3 elementary_test -to focus`

Reviewers: bu5hm4n, Jaehyun_Cho, zmike

Reviewed By: zmike

Subscribers: zmike, cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D11315