fix: frontmatter coercion (empty string is falsy)

Jacky Zhao

2023-12-09 2c69b0c97d40f76363d20478cb799ce4b7518e56

 quartz/util/path.test.ts

@@ -83,7 +83,7 @@
  test("simplifySlug", () => {
    asserts(
      [
        ["index", ""],
        ["index", "/"],
        ["abc", "abc"],
        ["abc/index", "abc/"],
        ["abc/def", "abc/def"],