Get flowBaseUri value only if in use

[kdambekalns]

Fixes #50

[mhsdesign]

not particularly pretty :D but that is due to flows tunnel through space which is not pretty either.

From you all id like to hear your opinion to neos/flow-development-collection#3334 (comment) which hopefully fixes this problem at its core by passing the base uri from a to z instead of putting it on a space station and have it accessible via worm hole :D

$this->resourceManager->getPublicPersistentResourceUri($resource, new Uri('http://localhost'));