fix: handle typed nil Stringers in ToStringE

[deepakganesh78]

Summary

• avoid calling String on typed nil pointer values implementing fmt.Stringer
• reuse the existing pointer-indirection helper without changing non-nil Stringer behavior
• cover custom pointer Stringers, *time.Time, and panic propagation from non-nil Stringers

Tests

• go test -shuffle on -v ./...
• go vet ./...
• golangci-lint run --new-from-rev=origin/master

Fixes #227

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