The PostgresVectorStore should have MetadataFilter for empty values

[kasteph]

From the docs, we can do:

...

filters = MetadataFilters(
    filters=[
        MetadataFilter(key="author", value="mats@timescale.com"),
        MetadataFilter(key="author", value="sven@timescale.com"),
    ],
    condition="or",
)

...

But we should also be able to do something like:

...

filters = MetadataFilters(
    filters=[
        MetadataFilter(key="author", value=None),
        MetadataFilter(key="author", value="sven@timescale.com"),
    ],
    condition="or",
)

...

where the value can be None since there are many cases in the real world where a filter can be None. For example:

1. Some documents might have no author attributed
2. Some documents might have some kind of other metadata such as university and others do not

and so on.