Accept empty string as initialization value

[limrv]

It reverts change from bd6bedc2.

Accepting blank strings is a more logical behavior since:

• Scalarless initialization values are currently accepted (Qty("m") => 1 m)
• Unitless initialization values are currently accepted (Qty("3") => 3)
• Units of a unitless quantity are an empty string (Qty("3").units() => "")
• Qty(qtyInstance.units()) is not expected to fail (it currently does with a unitless quantity)
• It is backward compatible with Ruby Units

[glepretre]