Avoid unnecessary ByteString concat for strict HTTP/2 entities

[He-Pin]

Motivation:
HTTP/2 DATA aggregation for small strict request entities may concatenate an empty ByteString with the incoming payload. That adds unnecessary work and allocation on unary/small-request gRPC-like paths without changing stream semantics.

Modification:

• keep the existing collecting state machine
• avoid collectedData ++ payload when either side is empty

Result:
The first DATA frame on the strict collection path now reuses the existing payload reference instead of allocating a concatenated ByteString. Non-empty aggregation behavior is unchanged.

Verification:

• sbt "http-core / scalafmtCheck" "http2-tests / Test / testOnly org.apache.pekko.http.impl.engine.http2.Http2ServerSpec -- -z collect"

Risk / Boundary:

• HTTP/2 stream state transitions are unchanged
• the change is limited to the empty/non-empty ByteString boundary
• existing HTTP/2 collection tests pass

[pjfanning]

ByteString ++ already checks if either the left or right bytestring is empty and then does not return a new ByteStrings instance.