Fields are not spreading north-west when they shouldn't anymore

[ZhilkinSerg]

Summary

SUMMARY: Bugfixes "Fields are not spreading north-west when they shouldn't anymore"

Purpose of change

Fixes #33275.

Describe the solution

When spread vector was empty, one_in( spread.size() ) would still be true as one_in( 0 ) returns true on any value less than or equal to 1, thus fields would still try spreading.

Spread direction should've been randomly selected using random_entry( spread ), but when collection is empty, random_entry just returns a default value for a given collection member type (0 for size_t).

That cause spreading to always choose neighs[ 0 ], which is a tile north-west of field initial tile.

I've added check for spread emptiness before trying to spread horizontally. Field would try to spread vertically if horizontal spread is impossible.