https://android-review.googlesource.com/#/c/67890/ removed field
initialization from the ctor, making it just call clear() instead.
When I added the generate_clear option back (as part of the reftypes
compat mode) in https://android-review.googlesource.com/#/c/109530/,
I forgot to ensure that what clear() used to do was inlined in the
constructor.
This change fixes NPEs that are happening for users of
reftypes_compat_mode who rely on unset repeated fields being empty
arrays rather than null.
Change-Id: Idb58746c60f4a4054b7ebb5c3b0e76b16ff88184