## Description
Blob mode cache entries (the default until a CL advertises
`engine_getBlobsV4` via `engine_exchangeCapabilities`) carry no cells,
but `Cache.GetCells` treated any entry as a hit and served null cells
from the empty custody set instead of falling back to the pool.
Found via the EELS execute-blobs simulator (EIP-8070 `engine_getBlobsV4`
tests), which never calls `engine_exchangeCapabilities`: null cells
appeared intermittently depending on the race with the 1s top-K preload.
With the fix the suite passes 139/139 (from 87/139 on master).
7a1b11564c