Make sure to set an entry page if none was set before (#10510)

* make sure to set an entry page if none is set yet

* fix possible error

* Site searches will set idaction=0, which is actually a valid entry page url, so we test specifically for false

* Add test case where the visit directly starts with an event

* Add system test where the first actions of a visit are events (without URL set), followed by a pageview later
-> the pageview will be set as the entry page

* Update processed api responses

* UI tests: two new fields in log_visit have been updated

* Processed API responses