In some cases German sentences don't need a subject, and this has nothing to do with passive voice or any prepositions:
Is a complete and absolutely correct German sentence without a subject. It just has an accusative object (mich) and a verb.
But sentences in passive voice often have the expletive pronoun »es« as subject:
Es wird gestreikt.
This is the same expletive »es« that you can find in:
It is raining.
and the English translation shows, that you have expletive pronouns in English as subjects too.
But over this, in German you can use this expletive pronoun in sentences too, that already contain an actor in nominative case:
Es ziehen Wolken über den Himmel. = Wolken ziehen über den Himmel.
There are clouds moving across the sky. = Clouds are moving across the sky.
As you see, English has a similar construction (not with it, but still similar).
But as said before, you often find this es in passive voice sentences:
active: Jemand streikt.
passive: Es wird gestreikt.
But now let's add a supplement to the predicate:
active: Jemand streikt seit gestern.
passive: Es wird seit gestern gestreikt.
This is correct, but now you also can move this supplement onto position 1, and now the expletive a placeholder for the subject is no longer necessary:
passive: Seit gestern wird gestreikt.
The expletive es is in singular, so the verb must be in singular too. This is true even if this es is replaced by a supplement to the predicate and therefore no longer exists. But its grammatical number still influences the verb.
The two sentences from your question are not exact active-passive-pairs. This are the correct pairs:
A: Seit gestern streiken die Mitarbeiter der Coca-Cola AG in Berlin.
P: Seit gestern wird in Berlin von den Mitarbeitern der Coca-Cola AG gestreikt.
But this passive construction is unusual, since it has no subject, but
a prepositional object starting with von. This combination is not wrong, but unusual.
P: Seit gestern wird bei der Coca-Cola AG gestreikt.
A: Jemand streikt seit gestern bei der Coca-Cola AG.
Here you need to invent a subject for the sentence in active voice, because the sentence in passive voice has no prepositional object starting with von (which would contain the actor).