[Gherkin] Fix NPE on empty table

[mpkorstanje]

Parsing the feature below results in an NPE because Gherkin v5 uses the location
of the first cell as the location of the table. Because there is no cell, there
is no location. In this case we fall back to using the line of the step + 1.

Feature: Empty table

  Scenario: This is valid Gherkin
    Given an empty list
      |

Fixes: #1912

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

[coveralls]

Coverage increased (+2.5%) to 85.528% when pulling 8ec0725 on gherkin-fix-npe-on-empty-table into 4d98e02 on master.