fix(DropdownToggle): fix 0 value #1496

[hedgecox]

• Bug fix
• New feature
• Chore
• Breaking change
• There is an open issue which this change addresses
• I have read the CONTRIBUTING document.
• My commits follow the Git Commit Guidelines
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• • I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

Replaced OR operator with null-coalescing operator to allow 0 value but not nullish values

#1496

[hedgecox]

Just seen the travis SyntaxError:
"Support for the experimental syntax 'nullishCoalescingOperator' isn't currently enabled (63:37)"

Will close this for now

[hedgecox]

Looking at the support it seems even though it's not fully supported in the browsers there is support for it in Babel: babel/proposals#14

Babel transpiles it as so:
const b = a ?? "else";
to
const b = a !== null && a !== void 0 ? a : "else";

However, this might be overkill for the problem at hand