evas textblock: Fix wrong bidi decision caused by empty text item

evas textblock: Fix wrong bidi decision caused by empty text item

An empty text item can be generated in the following squences.

ex) [LATIN]<preedit>[ARAB]</preedit>
1. Create a text item for LATIN script.
2. Create a text item for applying new format stage(<preedit>) to the next text.
3. Create a text item for ARAB script.
	=> The text item which is created from #2 will be remained empty.
	   And it will have 0 length, LATIN's bidi property and
	   same text_pos value with the item which is created from #3.

These empty text item can ruin bidi informations that is working based on text_pos(= cursor's position).

@tizen_fix

Change-Id: Iae219918e3acb1f3c4c6a8eec1f3e12bf77ae00b