fix ts error

lib/renderer.ts

@@ -33,7 +33,7 @@ function generateDoc(id: string) {
     })
 
     pdflatex.on('close', (code) => {
-      if (code > 0) {
+      if (code !== null && code > 0) {
         reject(`pdflatex returned with code ${code}`)
         return
       }