PageModelAggregationValve v1.0: Add a null check for component parameter values

Collectors.toMap doesn't accepts null values (https://bugs.openjdk.java.net/browse/JDK-8148463 ) and null check is needed in following method:

org.hippoecm.hst.pagemodelapi.v10.core.container.PageModelAggregationValve#getResidualParameters

Old 0.9 version doesn't throw the error. 

Note that the component parameter values can be null in case of an unresolved placeholder:
"If a propertyplaceholder can not be resolved, the entire parameter value is set to null." at
https://documentation.bloomreach.com/14/library/concepts/component-development/hstcomponent-configuration-properties.html