Date functions throw exceptions for blank and empty cells

[Brainman643]

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Do you want to request a feature or report a bug?

• Bug
• Feature
• Question

Did you test against the latest CI build?

• Yes
• No

Version of ClosedXML

0.104.0-preview3

What is the current behavior?

If the cell is blank date functions throw System.InvalidCastException with the message "Null object cannot be converted to a value type."
If the cell is an empty string date functions throw System.FormatException with the message "String '' was not recognized as a valid DateTime."

What is the expected behavior or new feature?

If the cell is blank date functions return the same value as if the cell had zero value.
If the cell is an empty string date functions return an "error" value.

Is this a regression from the previous version?

No, it's not. I also checked version 0.102.2.

Reproducibility

I wrote two tests.

Code to reproduce problem:

        [TestCase(31, "EOMONTH(A1,0)")] //System.InvalidCastException : Null object cannot be converted to a value type.
        [TestCase(5, "A1+5")] //Passed
        public void DateFormula_ReturnCorrectValue_WhenCellIsBlank(int expectedValue, string formula)
        {
            using (var wb = new XLWorkbook())
            {
                wb.ReferenceStyle = XLReferenceStyle.A1;

                var ws = wb.AddWorksheet("Sheet1");

                Assert.AreEqual(expectedValue, ws.Evaluate(formula).GetNumber());
            }
        }

        [TestCase("EOMONTH(A1,0)")] //System.FormatException : String '' was not recognized as a valid DateTime.
        [TestCase("A1+1")] //Passed
        public void DateFormula_ReturnError_WhenCellIsEmpty(string formula)
        {
            using (var wb = new XLWorkbook())
            {
                wb.ReferenceStyle = XLReferenceStyle.A1;

                var ws = wb.AddWorksheet("Sheet1");
                ws.Cell("A1").SetValue("");

                Assert.IsTrue(ws.Evaluate(formula).IsError);
            }
        }

• I attached a sample spreadsheet. (You can drag files on to this issue)
  In the file wb.xlsx cell A1 contains the formula for a blank cell, cell B1 contains the formula for an empty string.