Fix bug in Cadence Web where falsy values do not render correctly

[adhityamamallan]

Summary

This change fixes an issue with cadence-web where false/0 values in the activity result do not render correctly. This is due to a check in the getJsonStringObject helper function returning an empty string if the result is falsy. While this is the intended behaviour for null/undefined, we want to render other values (such as false and 0) correctly.

This change makes the check for null/undefined in the getJsonStringObject function more explicit.
As an aside, an unnecessary null check in the line below it is removed.

Test plan

Ran cadence-web locally and loaded a test workflow to ensure the output is as expected.

Before

After

To test null and undefined, I hardcoded the result:

To test the removal of the unnecessary null check, a unit test was added to ensure that the getStringElipsis util function returns the expected output.