Skip transfer if leaving rewards in prize pool

[trmid]

Use case:

• DrawManager is allocating the rest of the reserve to itself and wants to contribute it on behalf of a vault
• DrawManager "withdraws" the rewards, but sets the recipient as the prize pool, so no ERC20 token transfer needs to take place and the tokens stay where they are after the accounting updates.

[linear]

GEN-1262 Skip token transfer if withdrawRewards recipient is prize pool

[github-actions]

LCOV of commit 8b86ed1 during Tests with 100% Coverage #494

Summary coverage rate:
  lines......: 100.0% (424 of 424 lines)
  functions..: 100.0% (68 of 68 functions)
  branches...: no data found

Files changed coverage rate:
                                             |Lines       |Functions  |Branches    
  Filename                                   |Rate     Num|Rate    Num|Rate     Num
  =================================================================================
  src/PrizePool.sol                          | 100%    188| 100%    36|    -      0