Fixes #14390

Summary of the issue:
With the merging of pr #14070 which added sayall in tables, SayAll in Kindle for PC would fail to continue reading after turning the page.
Technical:
Some of the code in the nextLine method in sayAll was refacted into a nextLineImpl method.
However, if nextLineImpl returned false, finish would be called and nextLIne would return. This would be correct for handling the case where there was no more text, or table cells, but not the case where the page has just been turned. In fact, text sayAll already called finish when there was no more text, so calling finish again was useless in the base case, but for page turns caused sayAll to abort prematurely after the page was turned.

Description of user facing changes
NVDA no longer fails to keep reading with sayAll after crossing a page boundary.

Description of development approach
In sayAll's nextLine method, removed the call to self.finish when nextLineImpl returns False, but ensured that table sayAll's nextLineImpl does call finish itself if there is no more table cells.
In other words, the self.finish call has been moved into the specific nextLineImpl method where it is needed, rather than running more broadly.