Manipulation of expressions – Dimensions of the list used in the test.

I would like to reduce the number of parentheses in a list for nested lists that contain 1 sublist. For example {{1,2,3}} it must become {1,2,3}

But nevertheless, {{1,2,3}, {4,5}} I should stay the same.

Used to:

DropLevel[x_] : = Level[x, {-1}] /; Dimensions[x][[1]]== 1

What gives an error in {{1,2,3}, {4,5}} but he works in {{1,2,3}}

Dimensions[x][[1]]== 1 it works as a test to verify if a list has two secondary lists or only 1.

Am I missing something out of the way? /; functions?