fix: add nil check for OnTxStart before calling tracer hook

[Bue-von-hon]

Description

Calling vmCfg.Tracer.OnTxStart without a nil check causes a panic when a custom tracer does not implement the OnTxStart field. Added a nil guard consistent with the existing OnTxEnd check.

Closes #209

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Author Checklist

• tackled an existing issue or discussed with a team member
• left instructions on how to review the changes
• targeted the main branch

[greptile-apps]

Greptile Summary

This PR adds a nil guard for vmCfg.Tracer.OnTxStart in ApplyMessageWithConfig, preventing a panic when a custom tracing.Hooks instance leaves the OnTxStart function field unset. The fix is minimal, correct, and consistent with the existing nil check already applied to OnTxEnd in the same block.

• tracing.Hooks (from go-ethereum) is a struct with function-typed fields rather than an interface, so any field can be nil if a custom tracer doesn't populate it.
• The previous code called vmCfg.Tracer.OnTxStart(...) directly after confirming vmCfg.Tracer != nil, but never checked whether the OnTxStart field itself was non-nil — causing a nil-function-call panic for custom tracers that omit it.
• The added if vmCfg.Tracer.OnTxStart != nil guard mirrors the identical guard already present for OnTxEnd on the very next line.

Confidence Score: 5/5

• This PR is safe to merge — it is a targeted, one-line nil guard that closes a real panic path with no side effects.
• The change is a single conditional guard, directly mirrors an existing pattern in the same code block, and has no logic changes beyond preventing a nil function call dereference.
• No files require special attention.

Important Files Changed

Filename	Overview
x/vm/keeper/state_transition.go	Adds a nil guard for vmCfg.Tracer.OnTxStart before invoking it, matching the existing nil check for OnTxEnd. Correctly prevents a panic when a custom tracing.Hooks struct leaves OnTxStart unset (nil function field).

_{Last reviewed commit: e74482a}

[Bue-von-hon]

Please review.
I'm new to this and don't know who to mention, so I'm just leaving a comment here.