[Bug]: CSV File Input - unable to set constant as null value

### Apache Hop version?

2.9.0

### Java version?

openjdk version "21.0.2" 2024-01-16

### Operating system

Linux

### What happened?

In Hop GUI the CSV file input transform has no place to set whether a constant (such as "NULL") should be parsed as a null value (instead of a "NULL" string).

In the corresponding Metadata Injection view, there is a row labeled `Input fields -> Field null string`, but it seems ignored by Hop.

A partial workaround of the issue (does the job but still leaves unwanted traces in the logs, see #3893 for an example) is:
- setup CSV file input with `Lazy conversion` off, and to read all values as String
- link to a Null If transform, set the fields you need to evaluate and the constants to be parsed as null
- link to a Select values transform, in the Meta-data section specify the fields and their actual type and format (Integer, Date, etc.)

### Issue Priority

Priority: 3

### Issue Component

Component: Hop Gui, Component: Transforms

Start in the Hop GUI CSV file input transform and its Metadata Injection view, reproducing the case where a constant such as "NULL" should become a null value. Check why the `Input fields -> Field null string` setting is ignored; done means the configured constant is parsed as null without requiring the workaround transforms.

Written by the indexing model from the issue text.