tdf#89872, tdf#88547 use of date arrays as function arguments

Hi Eike,

The -now reverted- patch for tdf#88547 made it possible to convert an array of date strings to an 
array of numeric date values.
From tdf#88547 I learned that Excel accepts these arguments in functions like WORKDAY.INTL(), 
NETWORKDAYS.INTL(), MODE.MULT() and others. 
In https://bugs.documentfoundation.org/show_bug.cgi?id=89872#c3 you say
"The way how GetNumberSequenceArray() is used (and was implemented) it should ignore all string and 
empty values like all spreadsheet functions do that expect a number sequence as parameter.".

Now MODE.MULT() and the like do expect a number sequence as parameter _and_ are expected to accept 
arrays of date strings.

I want to fix 88547 in a way that doesn't break the use of GetNumberSequenceArray() as it did with 
my original patch, but I'm not sure which way to go.

Currently I think a proper solution would be to check whether we have a real non-empty string 
before we convert it to numeric and push an error when conversion to numeric fails; i.e. expand on 
my original patch of GetNumberSequenceArray().

Given that you know much more of this function and its purpose and history that I do (or can find 
out from git), I would like to hear your opinion/suggestion before I start coding.


Winfried