fix: resolved inconsistency for empty string csv input

[sfiorani]

This PR fixes a bug occurring in the CSV channels upload in the asset section.

If the CSV file contained an empty value for a field that should be casted in string format, it was treated as a "null" value, so the default one from the metatype was set. The correct behaviour should be that an empty csv value should be considered as an empty string, not a null value.

This change can be very impactful on the behavior of all those drivers/assets/channels that read variables in fields like String. Let’s say the case of a string field value which is then casted in the code to an integer with a Integer.decode: at the current state of the art, if this channel is load from csv null field, the validator would set this to a default value from the metatype (for example "10") which would then be casted into an integer; with the modification of this PR, the csv would be read as "" and then would fail the decode because you would try to cast an empty value as integer.

An example of this variation is depicted in the image below: the cannels were loaded from a CSV cotaining all the value field empty (first image). The driver in reading phase, reads the values set in the value channel field and tries to cast it to the value.type type. The metatype set the default value to 0, but passing a null value from the CSV, all values are considered as "", so the different casts fail (second image).

Related Issue: This PR fixes/closes {issue number}

Description of the solution adopted: The validator will now check which is the field type of the CSV field, and if it's a string of type "", it's admitted and not considered as a null value.

Screenshots:

Manual Tests: Tested on Raspberry Pi 3b, Kura version: generic-aarch64

Any side note on the changes made:

[MMaiero]

@sfiorani Do we have a way to load an empty string value from csvs?
Can we add a flag in the import modal to let the user select the preferred way to manage empty csv string values?
That would allow us to keep the consistency with the old behaviour

[sfiorani]

To mantain the compatibility with the previous behaviour, the following steps have been implemented:

• Added a checkbox in the csv upload popup that allow choosing if the old or new "empty value management" will be follow in the upload
• Added a short help box for the mentioned new checkbox
• Reorder of items in the popup
  So now it is possible to choose how to treat empty values in the csv when the field of interest is of String type

[sfiorani]

Tested also on Raspberry Pi, Kura version: raspberrypi-arm64

[MMaiero]

If checked, empty values contained in the csv will be parsed as empty strings, for all those fields whose type is

Maybe better like this: "If checked, all typed channel fields that have an empty value in the input csv will be parsed as empty strings."

[MMaiero]

Import empty values as empty strings.

Force import empty string policy

[MMaiero]

@sfiorani Did you already submit the updated documentation along with this PR?