mergeLocaleMessage. add changes notification when target obj is empty

[jekill]

add changes notification on merging with an empty target object

[codecov-io]

Codecov Report

Merging #752 into dev will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##              dev     #752   +/-   ##
=======================================
  Coverage   96.26%   96.26%           
=======================================
  Files          10       10           
  Lines         803      803           
=======================================
  Hits          773      773           
  Misses         30       30

Impacted Files	Coverage Δ
src/index.js	97.81% <100%> (ø)	⬆️

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[kazupon]

@jekill
Thank you for your PR!
Could you tell us the use-cases, please? 🙏

[jekill]

Yes, of course.
We have a situation like this.
When a file with translations for some locale is loaded asynchronously, at this time the i18n instance may or not may have translations for the locale. We always use merge i18n.mergeLocaleMessage but it doesn't always work the way we suggested.
For example:
In the case when we don't have i18n._vm.messages.ru
After i18n.mergeLocaleMessage('ru', loadedLocales), the field
i18n._vm.messages.ru becomes to contain loadedLocales,
but UI interface outs translation keys, not values because it doesn't get a notification that locales are changed

Here is a working example of this case
https://jsfiddle.net/ViktorK/o36vwcd5/20/

[kazupon]

OK, Thanks!