Allow zero as offset on <anchor> and <focus> inside <selection> (#3491)

* Allow zero as offset on <anchor> and <focus> inside <selection>

When creating a selection using the <selection> hyperscript tag, you couldn't
set zero as offset on <anchor> and <focus>, because of a faulty falsiness
check. This is fixed by checking if the offset is falsy and explicitly not zero.

* Make sure offset can't be null or undefined on <anchor> and <focus>

Throw exception when offset is strictly null or undefined, on <anchor> and
<focus> when creating a <selection>.

packages/slate-hyperscript/src/creators.ts

@@ -152,13 +152,13 @@ export function createSelection(
   const anchor: AnchorToken = children.find(c => c instanceof AnchorToken)
   const focus: FocusToken = children.find(c => c instanceof FocusToken)
 
-  if (!anchor || !anchor.offset || !anchor.path) {
+  if (!anchor || anchor.offset == null || anchor.path == null) {
     throw new Error(
       `The <selection> hyperscript tag must have an <anchor> tag as a child with \`path\` and \`offset\` attributes defined.`
     )
   }
 
-  if (!focus || !focus.offset || !focus.path) {
+  if (!focus || focus.offset == null || focus.path == null) {
     throw new Error(
       `The <selection> hyperscript tag must have a <focus> tag as a child with \`path\` and \`offset\` attributes defined.`
     )

packages/slate-hyperscript/test/fixtures/selection-offset-start.js

@@ -0,0 +1,35 @@
+/** @jsx jsx */
+
+import { jsx } from 'slate-hyperscript'
+
+export const input = (
+  <editor>
+    <element>word</element>
+    <selection>
+      <anchor path={[0, 0]} offset={0} />
+      <focus path={[0, 0]} offset={0} />
+    </selection>
+  </editor>
+)
+
+export const output = {
+  children: [
+    {
+      children: [
+        {
+          text: 'word',
+        },
+      ],
+    },
+  ],
+  selection: {
+    anchor: {
+      path: [0, 0],
+      offset: 0,
+    },
+    focus: {
+      path: [0, 0],
+      offset: 0,
+    },
+  },
+}