1.x: fix concatMap scalar/empty source behavior

[akarnokd]

There were two tiny problems with the rewritten concatMap operator, mainly due to copy-paste error:

• When a scalar was concatenated, the active field was not set and thus the next prefetched source overwrote it.
• When the scalar was set on the arbiter, its custom producer didn't check for n > 0 zero causing instant emission always. Generally Producer.request(0) is allowed and should be no-op (unlike RS).
• When an empty() was encountered as a last source, the it didn't trigger the check for completion afterwards.

[zsxwing]

👍

[stevegury]

👍