Handle composer.json file not existing

[cs278]

When adding a new composer.json file the diff fails as the file does not exist on the base branch, likewise the same would happen if the file were to be removed.

composer-rich-diff-action/src/diff.ts

Lines 100 to 118 in 73de1e2

	const base = this.octokit.rest.repos.getContent({
	owner: owner,
	repo: repo,
	path: path,
	ref: baseRef,
	mediaType: {
	format: "raw",
	},
	});
	const head = this.octokit.rest.repos.getContent({
	owner: owner,
	repo: repo,
	path: path,
	ref: headRef,
	mediaType: {
	format: "raw",
	},
	});