fix(store): make storePrefix actually optional

[Wondertan]

When we set the default value on the private config field while requiring it, we brake the serialization of the Parameters struct.
Particularly when we deserialize Parameters, the storePrefix is empty while the validation requires it. Because it's private, there is also no way to set it on the node, so this fix is necessary. This is also good learning that we should be cautious while introducing such optinonals

[codecov-commenter]

Codecov Report

Merging #69 (b389bbe) into main (0c1ebfb) will increase coverage by 0.24%.
The diff coverage is 70.73%.

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
+ Coverage   65.57%   65.81%   +0.24%     
==========================================
  Files          35       35              
  Lines        2806     2832      +26     
==========================================
+ Hits         1840     1864      +24     
- Misses        811      813       +2     
  Partials      155      155              

Impacted Files	Coverage Δ
store/options.go	54.05% <ø> (+2.83%)	⬆️
sync/sync.go	64.32% <ø> (ø)
sync/sync_head.go	62.40% <41.17%> (+0.36%)	⬆️
sync/sync_getter.go	90.47% <89.47%> (-9.53%)	⬇️
store/store.go	63.69% <100.00%> (+0.50%)	⬆️

... and 1 file with indirect coverage changes

[renaynay]

ugh