fix(buildkite): handle null build metadata without panicking

platform/buildkite/client.go

@@ -208,11 +208,9 @@ func ParseState(raw string) State {
 }
 
 // DecodeMetadataEnv recovers a JSON-encoded map[string]string from env[key].
-// Buildkite echoes env vars back on the build object, so a caller that seeded
-// a JSON-encoded map at Trigger time can recover it here at Status time
-// without any local state. Returns an empty non-nil map when the key is
-// absent. When the value is present but cannot be decoded, returns a nil map
-// and the decode error for the caller to log or act on as it sees fit.
+// Returns a non-nil map on success, including when the value is absent,
+// empty, or JSON null. A value that cannot be decoded returns a nil map
+// and the decode error.
 func DecodeMetadataEnv(env map[string]string, key string) (map[string]string, error) {
 	meta := make(map[string]string)
 	raw, ok := env[key]
@@ -222,5 +220,8 @@ func DecodeMetadataEnv(env map[string]string, key string) (map[string]string, er
 	if err := json.Unmarshal([]byte(raw), &meta); err != nil {
 		return nil, fmt.Errorf("decode metadata env %q: %w", key, err)
 	}
+	if meta == nil {
+		meta = make(map[string]string)
+	}
 	return meta, nil
 }

platform/buildkite/client_test.go

@@ -221,11 +221,27 @@ func TestDecodeMetadataEnv_PresentAndValid(t *testing.T) {
 	assert.Equal(t, map[string]string{"requester": "alice", "ticket": "SQ-42"}, got)
 }
 
-func TestDecodeMetadataEnv_Absent_ReturnsEmptyMap(t *testing.T) {
-	got, err := DecodeMetadataEnv(map[string]string{}, "SQ_METADATA")
-	require.NoError(t, err)
-	assert.Empty(t, got)
-	assert.NotNil(t, got)
+func TestDecodeMetadataEnv_Empty_ReturnsWritableMap(t *testing.T) {
+	tests := []struct {
+		name string
+		env  map[string]string
+	}{
+		{"absent", nil},
+		{"empty string", map[string]string{"SQ_METADATA": ""}},
+		{"empty object", map[string]string{"SQ_METADATA": "{}"}},
+		{"null", map[string]string{"SQ_METADATA": "null"}},
+		{"null with whitespace", map[string]string{"SQ_METADATA": " \nnull\t"}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := DecodeMetadataEnv(tt.env, "SQ_METADATA")
+			require.NoError(t, err)
+			require.NotNil(t, got)
+			assert.Empty(t, got)
+			got["url"] = "https://buildkite.com/test-org/my-pipeline/builds/42"
+			assert.Len(t, got, 1)
+		})
+	}
 }
 
 func TestDecodeMetadataEnv_Malformed_ReturnsError(t *testing.T) {