Check that changeset file exists

Author: hsubox76

.changeset/poor-llamas-boil.md

@@ -0,0 +1,5 @@
+---
+'@firebase/firestore': patch
+---
+
+Test

.changeset/shy-squids-thank.md

@@ -61,6 +61,10 @@ async function getDiffData(): Promise<{
 }
 
 async function parseChangesetFile(changesetFile: string) {
+  const fileExists = await fs.exists(changesetFile);
+  if (!fileExists) {
+    process.exit();
+  }
   const fileText: string = await fs.readFile(changesetFile, 'utf8');
   const fileParts = fileText.split('---\n');
   const packageLines = fileParts[1].split('\n');