Number formatting with a mask

Related to #614.
When using a mask (e.g. "$ #,##0") or currency format (eg., taken from docs: {locale: 'en-IN',options: { style:'currency', currency: 'INR' }}), if the first value entered in an empty cell is zero, the cell is blanked and the onafterchanges event does not fire. If you enter any non-zero value and then enter zero, the zero is retained.

You can see this behaviour in the jspreadsheet docs: https://bossanova.uk/jspreadsheet/docs/format.

In the "Currency Formatting" example - trying to enter 0 into column A demonstrates the described behaviour. Same lower down in the "Data Grid with Different Currencies" example. There are values with zero already present, so they will accept typing zero - but empty cells below will not unless you enter something else first.

Start by reproducing the issue in the Jspreadsheet documentation's Currency Formatting and Data Grid with Different Currencies examples at the linked format page. Trace the number-mask or currency-format handling for zero entered into an empty cell; done means zero remains visible and the onafterchanges event fires on the first entry.