Make field optional (to better reflect usage)

To be honest, TypeScript shouldn't have allowed the field to be
non-nullable to begin with.

Please see:

microsoft/TypeScript#17236

Author: start

src/Rendering/Html/HtmlRenderer.ts

@@ -214,7 +214,7 @@ export class HtmlRenderer extends Renderer {
     return this.htmlElement(
       'sup',
       [this.footnoteReferenceInnerLink(footnote)], {
-        id: this.footnoteReferenceHtmlId(footnote.referenceNumber),
+        id: this.footnoteReferenceHtmlId(footnote.referenceNumber!),
         class: cssClassAttrValue('footnote-reference')
       })
   }
@@ -346,16 +346,16 @@ export class HtmlRenderer extends Renderer {
     const { referenceNumber } = footnote
 
     return new Up.Link(
-      [new Up.Text(referenceNumber.toString())],
-      fragmentUrl(this.footnoteHtmlId(referenceNumber)))
+      [new Up.Text(referenceNumber!.toString())],
+      fragmentUrl(this.footnoteHtmlId(referenceNumber!)))
   }
 
   private footnoteInFootnoteBlock(footnote: Up.Footnote): string {
     const linkBackToReferenceContainer =
       this.htmlElement(
         'dt',
         [this.footnoteLinkBackToReference(footnote)],
-        { id: this.footnoteHtmlId(footnote.referenceNumber) })
+        { id: this.footnoteHtmlId(footnote.referenceNumber!) })
 
     const bodyContainer =
       this.htmlElement('dd', footnote.children)
@@ -367,8 +367,8 @@ export class HtmlRenderer extends Renderer {
     const { referenceNumber } = footnote
 
     return new Up.Link(
-      [new Up.Text(referenceNumber.toString())],
-      fragmentUrl(this.footnoteReferenceHtmlId(referenceNumber)))
+      [new Up.Text(referenceNumber!.toString())],
+      fragmentUrl(this.footnoteReferenceHtmlId(referenceNumber!)))
   }
 
   private playableMediaElement(playableMedia: Up.Audio | Up.Video, mediaTagName: string): string {

src/SyntaxNodes/Footnote.ts

@@ -4,7 +4,12 @@ import { Renderer } from '../Rendering/Renderer'
 
 
 export class Footnote extends RichInlineSyntaxNode {
-  referenceNumber: number
+  // Before the document is finalized, this field will always be undefined. Afterward, it will
+  // never be null.
+  //
+  // TODO: Research the best way to represent this using the type system. Should we create two
+  // versions of this class: One pre-finalization and one post-finalization?
+  referenceNumber?: number
 
   constructor(
     children: InlineSyntaxNode[],