Enforce non-null value from getBean and at injection points

Bean-derived null values may still get passed into bean properties and injection points but only if those are declared as non-required. Note that getBean will never return null; a manual bean.equals(null) / "null".equals(bean.toString()) check identifies expected null values now.  This will only ever happen with custom FactoryBeans or factory methods returning null - and since all common cases are handled by autowiring or bean property values in bean definitions, there should be no need to ever manually check for such a null value received from getBean.

Issue: SPR-15829

spring-beans/src/test/resources/org/springframework/beans/factory/config/PropertyPathFactoryBeanTests-context.xml

@@ -57,6 +57,16 @@
 		<property name="friends">
 			<bean name="otb.spouse" class="org.springframework.beans.factory.config.PropertyPathFactoryBean"/>
 		</property>
-  </bean>
+	</bean>
+
+	<bean id="tbWithNullReference" class="org.springframework.tests.sample.beans.TestBean">
+		<property name="spouse" ref="tb.spouse.spouse"/>
+	</bean>
+
+	<bean id="tbWithInnerNull" class="org.springframework.tests.sample.beans.TestBean">
+		<property name="spouse">
+			<bean name="tb.spouse.spouse" class="org.springframework.beans.factory.config.PropertyPathFactoryBean"/>
+		</property>
+	</bean>
 
 </beans>