Document filter_empty modifier

Closes #1495.

content/collections/modifiers/filter_empty.md

@@ -0,0 +1,23 @@
+---
+id: a01e28a5-7c59-436c-8137-98e9481631ba
+modifier_types:
+  - array
+title: 'Filter Empty'
+---
+Filters out null values from an array.
+
+```yaml
+favorite_things:
+  - pizza
+  - null
+  - ice cream
+```
+
+```antlers
+{{ favorite_things | filter_empty }}
+```
+
+```output
+pizza
+ice cream
+```