Improve correctness and error messages for JSON functions.

[nicktobey]

MySQL doesn't do this and neither should we.

MySQL:

mysql> select JSON_INSERT("null", "$.a", 1);
+-------------------------------+
| JSON_INSERT("null", "$.a", 1) |
+-------------------------------+
| null                          |
+-------------------------------+
1 row in set (0.00 sec)

mysql> select JSON_INSERT("null", "$.a", 1) is null;
+---------------------------------------+
| JSON_INSERT("null", "$.a", 1) is null |
+---------------------------------------+
|                                     0 |
+---------------------------------------+

The only time we should be coercing a JSON-null document into SQL-null is for JSON_EXTRACT (for paths other than "$") and JSON_VALUE (for all paths). But these are already handled separately.

[nicktobey]

So actually, it turns out that the only function that has any exceptional behavior around null documents is JSON_VALUE, which always returns SQL NULL for lookups on a document that is JSON NULL.

In all other cases, what we're seeing is the natural consequence of the fact that:

• Doing a lookup on a path that doesn't exist results in SQL NULL
• For a scalar document, the only valid lookup path is "$"

This PR now incorporates a couple of fixes that are just related enough that separating them into individual PRs would be a hassle:

• We're now stricter about types: most JSON functions require that the input is either a JSON column, or a string which can be coerced to JSON. Other types should result in an error, and should not be implicitly converted to JSON.
• Improved error messages to match MySQL when JSON functions are called with incorrect input.
• Avoid coercing JSON NULL to SQL NULL in the results of these functions: MySQL doesn't do it and neither should we.

There's another issue I encountered during this, which I documented but am not attempting to fix with this PR: dolthub/dolt#7905

[github-actions]

Additional work is required for integration with Dolt.
Additional work is required for integration with DoltgreSQL.

[github-actions]

Additional work is required for integration with Dolt.
Additional work is required for integration with DoltgreSQL.

[max-hoffman]

LGTM, i didn't individually check the errors against MySQL but the messages look more useful. I have also noticed weird path behavior, I haven't spent time trying to find an optimal solution but nudging in the right direction seems good.