Add default body content for root block

[firzanarmani]

Closes #426

If the blockTree is empty (i.e root has no children), show "Create a new note to get started!" Otherwise, show "Open a note to get started!"

[netlify[bot]]

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[codecov[bot]]

Codecov Report

Merging #427 into master will increase coverage by 0.13%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #427      +/-   ##
============================================
+ Coverage     66.83%   66.96%   +0.13%     
- Complexity      516      518       +2     
============================================
  Files            97       97              
  Lines          2014     2022       +8     
  Branches        201      201              
============================================
+ Hits           1346     1354       +8     
  Misses          580      580              
  Partials         88       88              

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/com/notably/model/block/BlockImpl.java	73.33% <100.00%> (ø)	6.00 <1.00> (ø)
...ava/com/notably/model/block/BlockTreeItemImpl.java	91.89% <100.00%> (+0.98%)	28.00 <3.00> (+2.00)

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