add action: uses table_concept_id if available

[ginberg]

see #50
add operation:

• use given concept_id if available
• extend current check for empty values
• add test

[ginberg]

could you try if this PR fixes #50 @laacri?

[laacri]

All works code-wise and the tests pass.

I replicated the gui scenario from issue #50 starting with a default observation period:

Then I assigned a condition occurrence by searching its concept, and it is added correctly with its id:

[laacri]

However, I noticed that if I try adding a second condition occurrence after that, the concept id of the new condition is assigned to the first one too:

If I try adding another one, the first one retains the id of the second which was assigned in the previous step, while the second one takes the id of the newly added one:

And so on if I keep adding condition occurrences. As for the dates, I just left the default ones for all of them. So the original problem with default ids is resolved, but there seems to be still something weird in the way the ids are updated when a new condition occurrence is added. I'm not sure if we want to address this in a new issue/branch