fix(ellipticalROI): give default values for cachedStats values to avoid undefined error

[md-prog]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Bug Fix:
_createTextBoxContent() function call during rendering sometimes causes undefined error.

• What is the current behavior? (You can also link to an open issue here)

Ideal flow would have been, first calculate all values of cachedStats and then call the _createTextBoxContent() function.
But in reality, particularly for the initial load, the _createTextBoxContent() function is called before the cachedStats values are calculated.

• What is the new behavior (if this is a feature change)?

Gave default value: 0, for the cachedStats values that are being used by _createTextBoxContent().
This will prevent undefined error until they are correctly caculcated.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

• Other information:

[codecov]

Codecov Report

Merging #1537 (0ada1b7) into master (925a3dd) will increase coverage by 1.22%.
The diff coverage is 58.82%.

❗ Current head 0ada1b7 differs from pull request most recent head d675825. Consider uploading reports for the commit d675825 to get more accurate results

@@            Coverage Diff             @@
##           master    #1537      +/-   ##
==========================================
+ Coverage   21.70%   22.92%   +1.22%     
==========================================
  Files         287      288       +1     
  Lines       10137    10070      -67     
  Branches     2081     2053      -28     
==========================================
+ Hits         2200     2309     +109     
+ Misses       6750     6611     -139     
+ Partials     1187     1150      -37     

Impacted Files	Coverage Δ
src/tools/annotation/_utils.js	48.14% <48.14%> (ø)
src/tools/annotation/CircleRoiTool.js	88.79% <100.00%> (+10.94%)	⬆️
src/tools/annotation/EllipticalRoiTool.js	75.92% <100.00%> (+42.59%)	⬆️
src/tools/annotation/RectangleRoiTool.js	84.00% <100.00%> (+40.60%)	⬆️

... and 6 files with indirect coverage changes

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[ladeirarodolfo]

LGTM

[sedghi]

🎉 This PR is included in version 6.0.9 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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