This commit avoids undefined.map() or null.map() is calling.
When there is no tag in a post, there are two cases in frontmatter.
1)
title: title
categories: Technical
...
2)
title: title
tags:
categories: Technical
...
Let's look at `post.node.frontmatter.tags` in both situations.
`FILE1 = /gatsby-node.js`
`FILE2 = /src/components/PostTemplateDetails/index.jsx`
In the first case, it is `undefined` in `FILE1`.
Cheking `undefined` is already implemented at FILE1#126, so undefined.map()
is not called.
But it is `null` in `FILE2#22`
and a crash will be occurred because it tries to run `null.map()`.
In the second case, it is `null` in `FILE1` and `FILE2`.
In this case, there will be two crashes in both files because both codes
try to run `null.map()`.
This commit avoids these crashes.
