net: lwm2m: Verify if block transfer is used before skipping TLV parsing

Verify if block tranfer is used and not an initial block when skipping
directly to data processing during FW update in PUSH mode.

This fixes a bug, which caused TLV not to be processed when the FW
object was updated as a whole, and actual resource number was encoded in
a TLV (for instance when writing FW Update URI).

Fixes #30135

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>