Treat empty strings as unparsable instead of open page ranges

Previously `pageLabelInRange` treated an empty label like a fully-open page
range ("-"). Since a fully open range overlaps all other ranges, the function
would always return true. Now it treats an empty string as an unparsable range
and returns false.

When filtering annotations, this change means that a page range filter will
no longer match annotations without a page range.

src/sidebar/util/page-range.ts

@@ -2,9 +2,20 @@
  * Parse a page number or range into a `[start, end]` integer pair. The `start`
  * or `end` may be `null` if the range is open.
  *
+ * Ranges can be specified as:
+ *
+ *  - A single number. This is treated as a range that starts and ends on the
+ *    same page.
+ *  - A range that may be closed ("5-10"), half-open ("5-" or "-5") or fully
+ *    open ("-").
+ *
  * Returns `null` if the page range could not be parsed.
  */
 function parseRange(range: string): [number | null, number | null] | null {
+  if (!range) {
+    return null;
+  }
+
   const [start, end] = range.includes('-') ? range.split('-') : [range, range];
 
   let startInt = null;

src/sidebar/util/test/page-range-test.js

@@ -92,6 +92,20 @@ describe('pageLabelInRange', () => {
       range: 'foo-bar',
       match: false,
     },
+
+    // Empty label
+    {
+      label: '',
+      range: '1-2',
+      match: false,
+    },
+
+    // Empty range
+    {
+      label: '1',
+      range: '',
+      match: false,
+    },
   ].forEach(({ label, range, match }) => {
     it('returns true if the label is in the page range', () => {
       assert.equal(pageLabelInRange(label, range), match);