Fixed start_date and end_date in user importer

[marcusmoore]

Description

This PR fixes an issue involving start and end dates in the user importer.

If a row being imported does not have a start or end date, whether that is due to it not being mapped or not being included in the import at all, the fields would end up in the database as 0000-00-00 instead of null. T

This is due to the findCsvMatch method returning an empty string if the column is present but empty and/or calling trim on null returning an empty string.

This PR simply changes start_date and end_date fields to null if they have been set to an empty string after the initial matching/mapping.

Fixes #15141

Type of change

• Bug fix (non-breaking change which fixes an issue)

[what-the-diff]

PR Summary

• Introduction of New Method for Managing Dates
  A new function handleEmptyStringsForDates() has been introduced to improve data handling. This function comes in handy when we encounter empty strings in our date fields, namely, start_date and end_date. Instead of keeping them as empty strings, which can lead to confusion and inconsistencies in our data, this method converts these empty strings to a neutral, more consistent value: null. This ensures a reliable, cleaner dataset which can enhance data analysis and result in more accurate insights.