Respect zero value

[pradpnayak]

Overview

If the value or weight for the option value is set to 0, the edit screen still recalculates the value or weight.

[civibot]

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[eileenmcnaughton]

@colemanw I've given this merge-ready just because you did tinker with it here https://github.com/civicrm/civicrm-core/pull/5538/files and I wanted to give you a chance to input.

I can see that

1. value could definitely legitimately be 0 - we just try to ensure value has 'something' and
2. it's a bit doubtful if 0 is legit for weight - but since it's not the default value it would only be 0 if delberately set to 0

[colemanw]

Seems fine to me.