chore: fix page title empty string

[nina992]

Overview

Page title can become empty string and then can't be selected anymore (should go back to UNTITLED)

What I've done

What I haven't done

How I tested

Which point I want you to review particularly

Memo

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[codecov]

Codecov Report

Merging #762 (bb7c018) into main (3df69f8) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #762      +/-   ##
==========================================
- Coverage   26.25%   26.25%   -0.01%     
==========================================
  Files        1610     1610              
  Lines      176189   176193       +4     
  Branches     3898     3900       +2     
==========================================
  Hits        46253    46253              
- Misses     128847   128851       +4     
  Partials     1089     1089              

Flag	Coverage Δ
web	24.54% <0.00%> (-0.01%)	⬇️
web-beta	24.54% <0.00%> (-0.01%)	⬇️
web-classic	24.54% <0.00%> (-0.01%)	⬇️
web-utils	24.54% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
.../Editor/tabs/story/LeftPanel/ContentPage/index.tsx	0.00% <0.00%> (ø)
.../lib/core/StoryPanel/Block/builtin/Title/index.tsx	0.00% <0.00%> (ø)

... and 3 files with indirect coverage changes