test: repro panic

[rootulp]

Closes #4847

[rootulp]

Finally able to repro the exact error stack trace

--- FAIL: TestSigVerificationDecorator (0.03s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x8 pc=0x1022e76e0]

goroutine 27 [running]:
testing.tRunner.func1.2({0x1044c6a60, 0x106f8d3f0})
	/Users/rootulp/sdk/go1.23.8/src/testing/testing.go:1632 +0x1bc
testing.tRunner.func1()
	/Users/rootulp/sdk/go1.23.8/src/testing/testing.go:1635 +0x334
panic({0x1044c6a60?, 0x106f8d3f0?})
	/Users/rootulp/sdk/go1.23.8/src/runtime/panic.go:791 +0x124
github.com/cosmos/cosmos-sdk/x/auth/tx.(*wrapper).GetSigningTxData(0x14001a24880)
	/Users/rootulp/go/pkg/mod/github.com/celestiaorg/cosmos-sdk@v1.29.1-sdk-v0.50.12/x/auth/tx/adapter.go:63 +0x540
github.com/cosmos/cosmos-sdk/x/auth/ante.SigVerificationDecorator.AnteHandle({{_, _}, _}, {{0x104bc9c30, 0x10706c840}, {0x104be9390, 0x140014c4d00}, {{0x0, 0x0}, {0x0, ...}, ...}, ...}, ...)
	/Users/rootulp/go/pkg/mod/github.com/celestiaorg/cosmos-sdk@v1.29.1-sdk-v0.50.12/x/auth/ante/sigverify.go:322 +0x734
github.com/celestiaorg/celestia-app/v4/app/ante_test.TestSigVerificationDecorator(0x14000492b60)
	/Users/rootulp/git/rootulp/celestiaorg/celestia-app/app/ante/ante_test.go:43 +0x39c
testing.tRunner(0x14000492b60, 0x104b60af0)
	/Users/rootulp/sdk/go1.23.8/src/testing/testing.go:1690 +0xe4
created by testing.(*T).Run in goroutine 1
	/Users/rootulp/sdk/go1.23.8/src/testing/testing.go:1743 +0x314
FAIL	github.com/celestiaorg/celestia-app/v4/app/ante	0.656s
FAIL