Fix nil errors when merging Taskfiles

Closes #150

internal/taskfile/merge.go

@@ -20,12 +20,24 @@ func Merge(t1, t2 *Taskfile, namespaces ...string) error {
 	if t2.Output != "" {
 		t1.Output = t2.Output
 	}
+
+	if t1.Includes == nil {
+		t1.Includes = make(map[string]string)
+	}
 	for k, v := range t2.Includes {
 		t1.Includes[k] = v
 	}
+
+	if t1.Vars == nil {
+		t1.Vars = make(Vars)
+	}
 	for k, v := range t2.Vars {
 		t1.Vars[k] = v
 	}
+
+	if t1.Tasks == nil {
+		t1.Tasks = make(Tasks)
+	}
 	for k, v := range t2.Tasks {
 		t1.Tasks[taskNameWithNamespace(k, namespaces...)] = v
 	}

task_test.go

@@ -484,3 +484,15 @@ func TestIncludes(t *testing.T) {
 	}
 	tt.Run(t)
 }
+
+func TestIncludesEmptyMain(t *testing.T) {
+	tt := fileContentTest{
+		Dir:       "testdata/includes_empty",
+		Target:    "included:default",
+		TrimSpace: true,
+		Files: map[string]string{
+			"file.txt": "default",
+		},
+	}
+	tt.Run(t)
+}

testdata/includes_empty/.gitignore

@@ -0,0 +1 @@
+file.txt

testdata/includes_empty/Taskfile.yml

@@ -0,0 +1,4 @@
+version: '2'
+
+includes:
+  included: Taskfile2.yml

testdata/includes_empty/Taskfile2.yml

@@ -0,0 +1,10 @@
+version: '2'
+
+vars:
+  FILE: file.txt
+  CONTENT: default
+
+tasks:
+  default:
+    cmds:
+      - echo "{{.CONTENT}}" > {{.FILE}}