Take into account that extractProofs might map over null

Per review comment by @XanderVertegaal in #79.

Author: jgonggrijp

frontend/src/app/services/parse.service.ts

@@ -37,7 +37,8 @@ export class ParseService {
  * the `proofs` and apply the `nltk2tableau` transformation to each proof
  * within.
  */
-function extractProofs(response: ParseResponse) {
+function extractProofs(response: ParseResponse | null) {
+    if (!response) return null;
     // This is basically just `_.mapObject(response.data.proofs, nltk2tableau)`,
     // but we don't depend on Underscore yet.
     const {entailment, contradiction} = response.data.proofs;