doubly nested cache table

This extra cache table should not be there:

```m2
Macaulay2, version 1.16.0.2
with packages: Colon, ConwayPolynomials, Elimination, IntegralClosure, InverseSystems, LLLBases, MinimalPrimes, PrimaryDecomposition, ReesAlgebra, TangentCone, Truncations

i1 : M = ZZ^2;

i2 : peek M.cache

o2 = CacheTable{cache => MutableHashTable{}}

i3 :

Process M2 finished

+ /Applications/Macaulay2-1.16/bin/M2 --no-readline --print-width 92
Macaulay2, version 1.16
with packages: ConwayPolynomials, Elimination, IntegralClosure, InverseSystems, LLLBases,
MinimalPrimes, PrimaryDecomposition, ReesAlgebra, TangentCone, Truncations

i1 : M = ZZ^2;

i2 : peek M.cache

o2 = CacheTable{cache => MutableHashTable{}}
```

Reproduce the report with `M = ZZ^2` followed by `peek M.cache`, comparing the two Macaulay2 outputs shown. Trace the cache handling behind this result and verify that the extra nested cache table is absent while the expected cache behavior remains intact.

Written by the indexing model from the issue text.