Resolve DIV/0 unstaking in a burn epoch with no burn

[EdNoepel]

Description of change

High level

Resolves bug if user unstakes following a burn epoch where nobody bid on the reserve auction.
Out of scope: Fuzz test improvements to capture scenario of 0 takes (this issue) or > 1 take.

Description of bug or vulnerability and solution

Resolves #744.

Contract size

Pre Change

============ Deployment Bytecode Sizes ============
  RewardsManager           -   9,509B  (38.69%)

Post Change

============ Deployment Bytecode Sizes ============
  RewardsManager           -   9,539B  (38.81%)

Gas usage

Slightly higher median cost due to 0 checks.

Pre Change

src/RewardsManager.sol:RewardsManager contract
Function Name	min	avg	median	max	# calls
calculateRewards	33603	45828	35922	163586	125
claimRewards	500	111048	104047	392830	128
getBucketStateStakeInfo	704	2702	2704	2704	81279
getStakeInfo	727	727	727	727	159
moveStakedLiquidity	1834728	1990365	1990365	2146003	2
stake	119064	346317	313046	894723	37
unstake	78743	180958	141392	398959	11
updateBucketExchangeRatesAndClaim	9680	254216	186927	537881	34

Post Change

src/RewardsManager.sol:RewardsManager contract
Function Name	min	avg	median	max	# calls
calculateRewards	33637	43022	35956	163688	291
claimRewards	500	112788	104081	392932	294
getBucketStateStakeInfo	704	2702	2704	2704	81279
getStakeInfo	727	727	727	727	334
moveStakedLiquidity	1834728	1990391	1990391	2146055	2
stake	119064	314061	197504	894723	46
unstake	78743	176425	141420	399137	13
updateBucketExchangeRatesAndClaim	9680	236554	176202	538191	44

[grandizzy]

LGTM

[MikeHathaway]

LGTM