empty? rewrite. (#2641)

docs/commands/empty.md

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+# empty?
+
+Check for empty values. Pass the column names to check emptiness. Optionally pass a block as the last parameter if setting contents to empty columns is wanted.
+
+## Examples
+
+Check if a value is empty
+```shell
+> echo '' | empty?
+true
+```
+
+Given the following meals
+```shell
+> echo [[meal size]; [arepa small] [taco '']]
+═══╦═══════╦═══════
+ # ║ meal  ║ size
+═══╬═══════╬═══════
+ 0 ║ arepa ║ small
+ 1 ║ taco  ║
+═══╩═══════╩═══════
+```
+
+Show the empty contents
+```shell
+> echo [[meal size]; [arepa small] [taco '']] | empty? meal size
+═══╦══════╦══════
+ # ║ meal ║ size
+═══╬══════╬══════
+ 0 ║ No   ║ No
+ 1 ║ No   ║ Yes
+═══╩══════╩══════
+```
+
+Let's assume we have a report of totals per day. For simplicity we show just for three days `2020/04/16`, `2020/07/10`, and `2020/11/16`. Like so
+```shell
+> echo [[2020/04/16 2020/07/10 2020/11/16]; ['' 27 37]]
+═══╦════════════╦════════════╦════════════
+ # ║ 2020/04/16 ║ 2020/07/10 ║ 2020/11/16
+═══╬════════════╬════════════╬════════════
+ 0 ║            ║         27 ║         37
+═══╩════════════╩════════════╩════════════
+```
+
+In the future, the report now has many totals logged per day. In this example, we have 1 total for the day `2020/07/10` and `2020/11/16` like so
+```shell
+> echo [[2020/04/16 2020/07/10 2020/11/16]; ['' [27] [37]]]
+═══╦════════════╦════════════════╦════════════════
+ # ║ 2020/04/16 ║ 2020/07/10     ║ 2020/11/16
+═══╬════════════╬════════════════╬════════════════
+ 0 ║            ║ [table 1 rows] ║ [table 1 rows]
+═══╩════════════╩════════════════╩════════════════
+```
+
+We want to add two totals (numbers `33` and `37`) for the day `2020/04/16`
+
+Set a table with two numbers for the empty column
+```shell
+> echo [[2020/04/16 2020/07/10 2020/11/16]; ['' [27] [37]]] | empty? 2020/04/16 { = [33 37] }
+═══╦════════════════╦════════════════╦════════════════
+ # ║ 2020/04/16     ║ 2020/07/10     ║ 2020/11/16
+═══╬════════════════╬════════════════╬════════════════
+ 0 ║ [table 2 rows] ║ [table 1 rows] ║ [table 1 rows]
+═══╩════════════════╩════════════════╩════════════════
+```
+
+Checking all the numbers
+```shell
+> echo [[2020/04/16 2020/07/10 2020/11/16]; ['' [27] [37]]] | empty? 2020/04/16 { = [33 37] } | pivot _ totals | get totals
+═══╦════
+ 0 ║ 33
+ 1 ║ 37
+ 2 ║ 27
+ 3 ║ 37
+═══╩════
+```