Fix nil errors when merging Taskfiles

Closes #150

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}}