Item preprocessing: Output condition is not applied if value does not match expression pattern

Steps:
1. Create host with item for monitoring apache server (web.page.get[localhost,/server-status])
2. Create dependent item with preprocessing configuration = “([0-9]*) day” and output =”0\1”
3. Restart apache server
4. Wait until items get information

Result: Dependent item is ‘Not supported’ bacause of:
Value "" of type "string" is not suitable for value type "Numeric (unsigned)"
Expected: Last value of dependent item equal to 0.