Check value emptiness in LocalTimePipe

[nealyip]

just like the way DateFormatPipe does,

if (!value) { return ‘’; }

[camillef]

Since amLocal currently always returns a Moment object, I'm not it should return '' if given an empty value.

Is the problem that https://github.com/urish/ngx-moment#amlocal-pipe returns 'Invalid date' when given an empty input? Maybe instead, amDateFormat should return '' if given an invalid Moment object