elm_entry: fix to keep api backward compatability

elm_entry: fix to keep api backward compatability

Summary:
if passing NULL as filename to elm_entry_file_set,
elm_entry_entry_get NULL as return value before.
and it doesn't now.

Test Plan:
1. passing a existing file to efl_entry_file_set
2. passing NULL as file to efl_entry_file_set
3. check return value of elm_entry_entry_get

Reviewers: zmike, bu5hm4n

Reviewed By: zmike

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D8363

Change-Id: I89fd726f7691f7105ec201b8b132fce546c38f58