Yes, when you use a CSV file, it is a little harder to pre-process so that empty values are handled the way you expect. But it is possible, note that all the examples below can be tried locally, just cut and paste them into local / feature files.

First let us see how to do this in a “normal” Examples block and see the difference later:

Feature: using nulls and inline json

  Scenario Outline:
    Given url 'https://reqres.in/api'
    And path 'register'
    And request { email: '#(email)', password: '#(password)' }
    When method post
    Then status 400
    * print response
    * match response == errorResponse

    Examples:
      | email! | password! | errorResponse!                                             |
      | null   | null      | { error: 'Missing email or username' }                     |
      | 'aa'   | 'bb'      | { error: 'Note: Only defined users succeed registration' } |

Note how the column names are suffixed with ! so that things like null are processed correctly instead of the default '' or empty-string. But when you use a CSV as the data-source, you need to do the pre-processing manually. Fortunately this is easy by using a @setup Scenario:

Feature: converting empty cells to null from a csv

  @setup
  Scenario:
    * def raw = read('examples.csv')
    * def convert = x => x || null
    * def data = raw.map(x => ({ email: convert(x.email), password: convert(x.password), error: x.error }))

  Scenario Outline:
    Given url 'https://reqres.in/api'
    And path 'register'
    And request { email: '#(email)', password: '#(password)' }
    When method post
    Then status 400
    * print response
    * match response == { error: '#(error)' }

    Examples:
      | karate.setup().data |

For the above example, the contents of examples.csv are the following:

email,password,error
,,Missing email or username
aa,bb,Note: Only defined users succeed registration

And for completeness, the other way of “looping” in the Karate world is this way:

Feature: looping over a json array converted from a csv

  Scenario:
    * def raw = read('examples.csv')
    * def convert = x => x || null
    * def data = raw.map(x => ({ email: convert(x.email), password: convert(x.password), error: x.error }))
    * call read('@register') data

  @ignore @register
  Scenario:
    Given url 'https://reqres.in/api'
    And path 'register'
    And request { email: '#(email)', password: '#(password)' }
    When method post
    Then status 400
    * print response
    * match response == { error: '#(error)' }