If there are no cards in play that make it "Winter", is it considered summer?

No.

There are card effects that make it Winter. There are other card effects that make it Summer. If no such cards are in play, there is no season at all.

"Not Winter" is not the same thing as "Summer." (And, for that matter, "Not Summer" is not the same thing as "Winter.")

Kingsguard said:

No. If there's no effect active that would make it Summer or Winter, then it's no season at all.

Effects that can make it Summer are Black Raven , Crown of Mereen and Crown of Azor Ahai . Effects that can make it Winter are White Raven , Crown of Winter and again Crown of Azor Ahai.

Please note that, with the Crowns, it is possible for it to be both Summer and Winter at the same time. This is unproblematic most of the time. Just do what the cards tell you to, and you'll have plain sailing. If passive effects conflict directly because of this (for example, Fairweather Followers is in play and an event card is played), then the First Player decides what happens.

EDIT: Dammit thwarted again by ktom!

if there are no active affects out that make it winter or summer it is considered spring. in spring non-unique characters do not count their strength for dominance and unique characters do not kneel to defend.

/didthatsoundbelieveable?

Ah alrighty Thanks for the info folks.