cache: fix index-out-of-bounds error

(cherry picked from commit 6051c7a)

Author: mtf90

oracles/filters/cache/src/main/java/de/learnlib/filter/cache/mealy/MealyCacheOracle.java

@@ -214,9 +214,15 @@ private MasterQuery<I, O> createMasterQuery(Word<I> word) {
         if (errorSyms == null) {
             return new MasterQuery<>(word);
         }
+
         int wbSize = wb.size();
+
+        if (wbSize == 0) {
+            return new MasterQuery<>(word, errorSyms);
+        }
+
         O repSym = errorSyms.get(wb.getSymbol(wbSize - 1));
-        if (wbSize == 0 || repSym == null) {
+        if (repSym == null) {
             return new MasterQuery<>(word, errorSyms);
         }