fixing nil pointer exception for Getters

[Chengxuan]

The get methods are not parsing the retrieved value, instead, they call Get method to retrieve the value again.

This caused nil pointer exception when the cache value had expired or been deleted.

2026-06-29T12:26:35.548Z] ERROR observed panic (execMapped) 'interface conversion: interface {} is nil, not string': goroutine 433125 [running]:
runtime/debug.Stack()
	/usr/local/go/src/runtime/debug/stack.go:26 +0x5e
github.com/kaleido-io/workflow-engine/pkg/enginesdk.execMapped[...].func1()
	/workflow-engine/pkg/enginesdk/txn_handler.go:442 +0x7c
panic({0x1af4920?, 0xc000cc05d0?})
	/usr/local/go/src/runtime/panic.go:783 +0x132
github.com/hyperledger/firefly-common/pkg/cache.(*CCache).GetString(0xc00047bef0, {0xc000c69c00, 0x3a})
	/go/pkg/mod/github.com/hyperledger/firefly-common@v1.5.11-0.20260428164525-50bebf579bab/pkg/cache/cache.go:127 +0x77

[EnriqueL8]

Good catch!