fix: avoid panic when ToStringE receives nil Stringer pointer

[leno23]

Summary

• Fixes a panic when ToStringE is called with a nil pointer whose type implements fmt.Stringer (reported for *time.Time in Bug or bad entry ? #227)
• Guard the fmt.Stringer branch so nil pointer/interface receivers return "" instead of calling String() on a nil receiver

Test plan

• Added nil *time.Time case to TestString
• go test ./...
• Manual repro: cast.ToStringE((*time.Time)(nil)) returns "" without panic

Fixes #227

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