nullValue does not output "null" string

[arina-ielchiieva]

!set nullValue null
values (null, -1.5);

Result:

+----+------+
| C1 |  C2  |
+----+------+
|    | -1.5 |
+----+------+

Though if to set null value in camel case everything works as expected:

!set nullValue Null
values (null, -1.5);

Result:

+------+------+
|  C1  |  C2  |
+------+------+
| Null | -1.5 |
+------+------+

Issue is mostly likely because when comparing property default and current value, these values are compared as strings but null and "null" are different values.

https://github.com/julianhyde/sqlline/blob/master/src/main/java/sqlline/SqlLineOpts.java#L410

[julianhyde]

I have never liked the idea of using "null" as a special value in !set. Or the idea of comparing with default values.

Instead, I think we should have an !unset command. Then "null" would no longer have a special meaning; it would just be a string with 4 characters.

[arina-ielchiieva]

If nullValue is not set / default, nulls are output as empty strings. Setting "null" value to nullValue property allowed to output nulls as "null" (worked in 1.6, broke in 1.7). We do have !reset command which can reset properties to their defaults but still nulls will be output as empty strings in this case.
PR with the fix - #300.

[julianhyde]

Fixed in a689205, PR #300. Thanks @arina-ielchiieva!