Hide drop empty rows checkbox when no file is selected in import dataset dialog

[mmulahvictor]

Fixes #7173

• I have hidden the drop checkbox when no file is selected.
• I have disable ok which appeared after reseting the dialog.

@africanmathsinitiative/developers
This is ready for review

[rdstern]

This is a nice example of a simple task where I would like the programmer, here @mmulahvictor to do more testing and also to read the task carefully, before sending it for review.
a) When I press Reset as suggested by @shadrackkibet it is perfect and all works fine. But when I return to the dialogue again, the option has returned. So it isn't obeying the rule that the dialogue re-opens as it was left.
b) Shadrack suggested the message on not displaying files of this type could/should(?) also be changed. It isn't yet. Either there should be a comment that this remains to be done, or that the programmer doesn't agree, and so decided not to do it.

[mmulahvictor]

@N-thony made the changes for the message on not displaying files of this type in the database.

[lloyddewit]

@mmulahvictor when PR #7243 is merged, please could you pull the changes into this PR so that the new text can also be tested, thanks

[mmulahvictor]

@shadrackkibet can you look at the changes I have made above?

[shadrackkibet]

@Ivanluv could you review the following PR?

[lloyddewit]

@rdstern please could you test? thanks