Solr的WiKI原文是如下解释：
facet.missing
Set to "true" this param indicates that in addition to the Term based constraints of a facet field, a count of all matching results which have no value for the field should be computed
The default value is false.
This parameter can be specified on a per field basis.
个人实验下来，当missing=true时，只有当facet.field在匹配的document中根本不存在（就是在生成索引时，根本就没有加入这个field），才会被统计。（即使field为null或者为“”，都不会被统计）。
