blank() filter (#226)

Added blank filter based on:
https://github.com/syslog-ng/syslog-ng/pull/5425/files

Author: zsoltgyulai94

doc/_admin-guide/080_Log/030_Filters/005_Filter_functions/013_blank.md

@@ -0,0 +1,23 @@
+---
+title: blank()
+id: adm-log-filters-blank
+description: >-
+    This section describes blank() filter function in {{ site.product.short_name }}.
+---
+
+*Synopsis:* blank("MESSAGE")
+
+
+*Description:* The blank() filter selects messages where the specified key-value pair is considered blank. A value is considered blank if it meets any of the following conditions:
+
+* It is not set
+* It is an empty string ("")
+* It contains only whitespace characters
+* It is an empty list
+* It is a boolean value explicitly set to false
+
+## Example: Select log messages where the field MESSAGE is blank
+
+```config
+blank("MESSAGE")
+```

doc/_admin-guide/080_Log/030_Filters/005_Filter_functions/README.md

@@ -24,3 +24,5 @@ described in Filters.
 |rate-limit() |Limits messages rate based on arbitrary keys in each message. |
 |source()|Select messages of the specified {{ site.product.short_name }} source statement.|
 |tags()|Select messages having the specified tag.|
+|blank()|Filter messages where the specified value is blank (unset, empty, whitespace-only, empty list, or boolean false).|
+