Fix config handling with empty config entries after merge

Fixes #8701

config/defaultConfigProvider.go

@@ -214,6 +214,7 @@ func (c *defaultConfigProvider) Merge(k string, v interface{}) {
 		if p, ok := maps.ToParamsAndPrepare(v); ok {
 			// As there may be keys in p not in root, we need to handle
 			// those as a special case.
+			var keysToDelete []string
 			for kk, vv := range p {
 				if pp, ok := vv.(maps.Params); ok {
 					if pppi, ok := c.root[kk]; ok {
@@ -261,14 +262,19 @@ func (c *defaultConfigProvider) Merge(k string, v interface{}) {
 						strategy := c.determineMergeStrategy(KeyParams{Key: "", Params: c.root}, KeyParams{Key: kk, Params: np})
 						np.SetDefaultMergeStrategy(strategy)
 						np.Merge(pp)
-						if len(np) > 0 {
-							c.root[kk] = np
+						c.root[kk] = np
+						if np.IsZero() {
+							// Just keep it until merge is done.
+							keysToDelete = append(keysToDelete, kk)
 						}
 					}
 				}
 			}
 			// Merge the rest.
 			c.root.Merge(p)
+			for _, k := range keysToDelete {
+				delete(c.root, k)
+			}
 		} else {
 			panic(fmt.Sprintf("unsupported type %T received in Merge", v))
 		}

config/defaultConfigProvider_test.go

@@ -283,6 +283,26 @@ func TestDefaultConfigProvider(t *testing.T) {
 
 	})
 
+	// Issue #8701
+	c.Run("Prevent _merge only maps", func(c *qt.C) {
+		cfg := New()
+
+		cfg.Set("", map[string]interface{}{
+			"B": "bv",
+		})
+
+		cfg.Merge("", map[string]interface{}{
+			"c": map[string]interface{}{
+				"_merge": "shallow",
+				"d":      "dv2",
+			},
+		})
+
+		c.Assert(cfg.Get(""), qt.DeepEquals, maps.Params{
+			"b": "bv",
+		})
+	})
+
 	c.Run("IsSet", func(c *qt.C) {
 		cfg := New()