fix: When definition doesn't conform don't error out, return null

src/Entity/Term.php

@@ -67,7 +67,12 @@ class Term
             $term->kanji = mb_substr($vocabDefLine, 0,            $enStart);
             $def         = mb_substr($vocabDefLine, $enStart + 1, null);
             $enStart = 0;
+        } else {
+            // Can't extract term from definition, it doesn't conform to the
+            // established pattern.
+            return null;
         }
+
         // Convert 「this」 into [this]
         $term->kanji = mb_trim(strtr($term->kanji, [
             '「' => '[',