FIX getCachedMarkup returns NULL instead of string if file doesn't exist

[sabina-talipova]

Description

• Check if file exist before return cached link for this file from getCachedMarkup method.
• Method handle_shortcode should return string.

Test steps

• Set FileShortcodeProvider::allow_session_grant = true;
• Create new Page or open existing one in CMS;
• Click Insert Link to a file in Content field;
• Select any existing file and Save page;
• Remove file from backend or change link in DB record;
• Open Page in CMS again;
• You should not see any Errors;
• Shortcode link should lead to "Home" page;

Parent issue

• Error when previewing page and asset doesn't exist #545

[GuySartorelli]

The bug was reported for CMS 4 - please retarget this PR to the 1.13 branch.