null-check value in getGeneratorDefinition

Tests were failing. Also, assert the last return is a GeneratorDefinition.

I have to clean this up, but essentially TS is being unable to narrow
this down. An InputOutputGeneratorOptions is a one level deep object
with GeneratorOptions, so the recursive call will inevitably trickle
down to a base case where we _do not_ have an InputOutputGeneratorOptions.

Author: argvniyx-enroute

src/index.ts

@@ -102,11 +102,11 @@ const getGeneratorDefinition = (
         };
     }
 
-    if ('input' in value && 'output' in value) {
+    if (value !== undefined && 'input' in value && 'output' in value) {
         return getGeneratorDefinition(value[generatorMode], generatorMode);
     }
 
-    return value;
+    return value as GeneratorDefinition;
 };
 
 const getCasualCustomValue = (