refactor: Remove redundant nil check for slice length in dummy_test.go (#2333)

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## Overview

I forked this project and added lint checks in the CI, using staticcheck
to identify this issue.

```shell
rollkit/core/execution/dummy_test.go:30:5: should omit nil check; len() for nil slices is defined as zero (S1009)
```


In Go, `len()` on a nil slice returns 0. Therefore, explicitly checking
if a slice is `nil` before checking its length (e.g., `if s != nil &&
len(s) > 0`) is redundant. This commit removes the unnecessary `nil`
check for `executor.injectedTxs` in the `TestNewDummyExecutor` function
in <mcfile name="dummy_test.go"
path="core/execution/dummy_test.go"></mcfile>, simplifying the condition
to `if len(executor.injectedTxs) > 0`. This change adheres to common Go
idioms and improves code readability without altering behavior.



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<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

- **Refactor**
- Simplified logic for checking if injected transactions are present in
tests.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: clonemycode <clonecode@aliyun.com>

Author: clonemycode

core/execution/dummy_test.go

@@ -27,7 +27,7 @@ func TestNewDummyExecutor(t *testing.T) {
 		t.Error("pendingRoots map was not initialized")
 	}
 
-	if executor.injectedTxs != nil && len(executor.injectedTxs) > 0 {
+	if len(executor.injectedTxs) > 0 {
 		t.Errorf("Expected injectedTxs to be empty, got %v", executor.injectedTxs)
 	}
 }