- added test for issue #7550
- solved issue #7550
- implemented getPrimaryKey member function of QgsSpatiaLiteProvider
- in addFeatures, if an attribute is a primary key, its value is set to
  NULL so that sqlite generates a value automagically
Merge remote-tracking branch 'upstream/master'
- removed useless function getPrimaryKey since a member already exist
  for that
- modified the test to have multiple column primary key (it fails for
  the moment
- solves issue #7550
- the feature modification stops (in edit buffer) once an error is found
- the test has been modified to refelect the real problem: the data
  corruption, and not the commit failure if primary key are not unique
- set primary key value to NULL in split when there is no default value.
- modified the test accordingly
Merge pull request #715 from Oslandia/issue7550

solves Issue7550