fix: handle nil values in concatMaps to prevent SIGSEGV

[lxcxjxhx]

Description

Fix a nil pointer dereference (SIGSEGV) panic in concatMaps when merging map[string]any stream chunks where a key is nil in an earlier chunk and set in a later chunk.

Root Cause

toSliceValue calls reflect.TypeOf(vs[0]) where vs is a []any. When the first element is nil, reflect.TypeOf returns nil, causing reflect.SliceOf(nil) to panic with SIGSEGV.

This happens in concatMaps at line 151 when collecting per-key values across chunks — nil values from map entries get appended to the []any slice via reflect.Append, and toSliceValue doesn't handle nil elements.

Fix

In toSliceValue, filter out nil elements before processing. If all elements are nil, return an empty []any slice. Otherwise, use the first non-nil element's type for the slice type. This makes concat behavior symmetric regardless of whether nil values appear before or after non-nil values.

Changes

• internal/concat.go: Handle nil elements in toSliceValue
• internal/concat_test.go: Add test cases for nil-first, nil-last, and mixed-nil scenarios

Testing

• go test ./internal/... -count=1 — all pass
• Reproducer from issue: ConcatItems([]map[string]any{{"key": nil}, {"key": "value"}}) no longer panics

Related

• Fixes [internal/concat] ConcatItems panics (SIGSEGV) when a map[string]any stream key is nil in an earlier chunk and set in a later chunk #1181