Add A Better Mull Display Character as postgres til.

postgres/a-better-null-display-character.md

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+# A Better Null Display Character
+
+By default, `psql` will display null values with whitespace. This makes it
+difficult to quickly identify null values when they appear amongst a bunch
+of other data. You can pick a better display value for null characters with
+`\pset null`. My preference is the following:
+
+```
+\pset null 'Ø'
+```
+
+I have this in my `.psqlrc` file so that it is used by default every time.