Hello!
The solution is non-trivial because imported_1 will return an empty space (not a zero) if you leave it out in the text file.
So you need to insert this formula into the Amount field:
IF(imported_1+0 > 0.05, imported_1+0, default_backa)
The trick here is that is imported_1 is empty, the expression will be down to:
IF(+0 > 0.05, +0, default_backa)
which is a perfectly valid trigger expression, and it will return default_backa.
On the other hand, if imported_1 has some value, e.g. 3.5, then 3.5 + 0 will still be 3.5, so this won't affect your staking plan.