Emty-ish values parsing

[jansiegel]

HyperFormula is parsing Handsontable's commonly used empty-ish values in a way that makes them unusable in the table.

For example, passing a dataset/sheet as:
[['', ' ']] _{(empty string, space)}
translates to:
[Symbol(), 0]

and with a null value we get the following error:

Uncaught TypeError: Cannot read property 'length' of null
    at Object.isMatrix (ParserWithCaching.ts:123)
    at HyperFormula.setMultipleCellContents (HyperFormula.ts:258)

(similar in case of undefined)

[swistak35]

I've added possibility to handle null and undefined values. Do we want to handle numbers too (imho if we expanded to null/undefined, it make sense)?

Test:

hyperformula/test/engine.spec.ts

Lines 37 to 43 in 299d21c

	it('handle different input types', () => {
	const engine = HyperFormula.buildFromArray([['', null, undefined]])
	expect(engine.getCellValue(adr('A1'))).toBe(EmptyValue)
	expect(engine.getCellValue(adr('B1'))).toBe(EmptyValue)
	expect(engine.getCellValue(adr('C1'))).toBe(EmptyValue)
	})

Also I've fixed bug with " " being understood as number 0.

Test:

hyperformula/test/CellContentParser.spec.ts

Line 43 in 299d21c

expect(cellContentParser.parse(' ')).toStrictEqual(new CellContent.String(' '))

[voodoo11]

After some arrangements:

1. Empty string should be treated as CellValue
   1.1. getCellValue on such cell will return empty string
   1.2. getCellValueType will return CellValueType.STRING

2. null will be treated as EmptyCellValue
   1.1. getCellValue will return EmptyValue
   1.2. getCellValueType will return CellValueType.EMPTY

[swistak35]

Empty string should be treated as CellValue

I think that if we are going this way, then CsvImporter for empty fields, should change empty strings to null, before constructing HyperFormula. I.e. We'd expect such CSV: ,=ISBLANK(A1) to have true in B1?

[voodoo11]

Done #46