Keep module context data when a module's first hook returns none

hooks/hookexecution/context.go

@@ -54,7 +54,7 @@ func (mc *moduleContexts) put(moduleName string, mCtx *hookstage.ModuleContext)
 	defer mc.Unlock()
 
 	existingCtx, ok := mc.ctxs[moduleName]
-	if !ok {
+	if !ok || existingCtx == nil {
 		mc.ctxs[moduleName] = mCtx
 		return
 	}

hooks/hookexecution/context_test.go

@@ -0,0 +1,39 @@
+package hookexecution
+
+import (
+	"testing"
+
+	"github.com/prebid/prebid-server/v4/hooks/hookstage"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestModuleContextsPutAfterNil(t *testing.T) {
+	mcs := &moduleContexts{ctxs: map[string]*hookstage.ModuleContext{}}
+	mcs.put("module", nil)
+
+	later := hookstage.NewModuleContext()
+	later.Set("key", "value")
+	mcs.put("module", later)
+
+	got, ok := mcs.get("module")
+	assert.True(t, ok)
+	v, found := got.Get("key")
+	assert.True(t, found)
+	assert.Equal(t, "value", v)
+}
+
+func TestModuleContextsPutMergesAndKeepsNilHarmless(t *testing.T) {
+	mcs := &moduleContexts{ctxs: map[string]*hookstage.ModuleContext{}}
+
+	first := hookstage.NewModuleContext()
+	first.Set("a", 1)
+	mcs.put("module", first)
+	mcs.put("module", nil)
+
+	second := hookstage.NewModuleContext()
+	second.Set("b", 2)
+	mcs.put("module", second)
+
+	got, _ := mcs.get("module")
+	assert.Equal(t, map[string]any{"a": 1, "b": 2}, got.GetAll())
+}