fix: when schema is empty getFieldType throw error with nestedObject (#84)

Author: myNameIsDu

src/MixedType.ts

@@ -69,7 +69,7 @@ export function getFieldType(schemaSpec: any, fieldName: string, nestedObject?:
           return type;
         }
 
-        if (namePath.length > 1 && type[schemaSpecKey]) {
+        if (namePath.length > 1 && type && type[schemaSpecKey]) {
           return getFieldType(type[schemaSpecKey], namePath.slice(1).join('.'), true);
         }
       }

test/MixedTypeSpec.js

@@ -1206,6 +1206,7 @@ describe('#MixedType', () => {
       expect(getFieldType(null, 'a')).to.be.undefined;
       expect(getFieldType(undefined, 'a')).to.be.undefined;
       expect(getFieldType({}, 'a')).to.be.undefined;
+      expect(getFieldType({}, 'a.a[1]', true)).to.be.undefined;
       expect(getFieldType(schema, '')).to.be.undefined;
       expect(getFieldType(schema, '..')).to.be.undefined;
       expect(getFieldType(schema, 'a.b.c[0].d', true)).to.be.undefined;