Parent implementation of transformDimensions breaks dimensions array, responsive image srcset attributes

Problem/Motivation

MaxSizeCropImageEffect does not implement transformDimensions so parent implementation from ResizeImageEffect is used, which assumes both width and height configuration parameters will be present. However this isn't the case in this plugin so if one of the values is left blank then the corresponding entry in $dimensions will be set to an empty value. In the case of width being empty, this confuses things like the responsive image formatter which needs non-empty width values with when building srcset attributes.

Proposed resolution

Implement transformDimensions so $dimensions entries are only modified if the corresponding configuration values are not empty.