presets: Do not add "custom_tags: {}" to clusters (#2838)

Follow up to #2756

Author: denik

acceptance/bundle/override/clusters/output.txt

@@ -6,7 +6,6 @@
     "min_workers": 2
   },
   "cluster_name": "foo",
-  "custom_tags": {},
   "node_type_id": "i3.xlarge",
   "num_workers": 2,
   "spark_conf": {
@@ -22,7 +21,6 @@
     "min_workers": 1
   },
   "cluster_name": "foo-override",
-  "custom_tags": {},
   "node_type_id": "m5.xlarge",
   "num_workers": 3,
   "spark_conf": {

acceptance/bundle/override/merge-string-map/output.txt

@@ -7,7 +7,6 @@ Warning: expected map, found string
 {
   "clusters": {
     "my_cluster": {
-      "custom_tags": {},
       "spark_version": "25"
     }
   }

acceptance/bundle/validate/empty_resources/empty_dict/output.txt

@@ -86,9 +86,7 @@
 === resources.clusters.rname ===
 {
   "clusters": {
-    "rname": {
-      "custom_tags": {}
-    }
+    "rname": {}
   }
 }
 

acceptance/bundle/validate/empty_resources/with_grants/output.txt

@@ -115,9 +115,7 @@ Warning: unknown field: grants
 
 {
   "clusters": {
-    "rname": {
-      "custom_tags": {}
-    }
+    "rname": {}
   }
 }
 

acceptance/bundle/validate/empty_resources/with_permissions/output.txt

@@ -103,7 +103,6 @@ Warning: unknown field: permissions
 {
   "clusters": {
     "rname": {
-      "custom_tags": {},
       "permissions": []
     }
   }

bundle/config/mutator/resourcemutator/apply_presets.go

@@ -195,14 +195,16 @@ func (m *applyPresets) Apply(ctx context.Context, b *bundle.Bundle) diag.Diagnos
 			continue
 		}
 		c.ClusterName = prefix + c.ClusterName
-		if c.CustomTags == nil {
-			c.CustomTags = make(map[string]string)
-		}
-		for _, tag := range tags {
-			normalisedKey := b.Tagging.NormalizeKey(tag.Key)
-			normalisedValue := b.Tagging.NormalizeValue(tag.Value)
-			if _, ok := c.CustomTags[normalisedKey]; !ok {
-				c.CustomTags[normalisedKey] = normalisedValue
+		if len(tags) > 0 {
+			if c.CustomTags == nil {
+				c.CustomTags = make(map[string]string, len(tags))
+			}
+			for _, tag := range tags {
+				normalisedKey := b.Tagging.NormalizeKey(tag.Key)
+				normalisedValue := b.Tagging.NormalizeValue(tag.Value)
+				if _, ok := c.CustomTags[normalisedKey]; !ok {
+					c.CustomTags[normalisedKey] = normalisedValue
+				}
 			}
 		}
 	}