Implementations must return the primary BeanDefinition that results
from the parse if they will ever be used in a nested fashion (for example as
an inner tag in a <property/> tag). Implementations may return
null if they will not be used in a nested fashion.
element - the element that is to be parsed into one or more BeanDefinitions
parserContext - the object encapsulating the current state of the parsing process;
provides access to a BeanDefinitionRegistry