Support CREATE TABLE unique keys with no key name

[akirk]

In a table like

CREATE TABLE `test` (
	`id` INT,
	`name` VARCHAR(255),
	`other` VARCHAR(255),
	PRIMARY KEY (id),
	UNIQUE KEY (name)
)

the internal name for the unique key would be ( because typically a key would be defined with

UNIQUE KEY `name` (name)

Usually this wrong key name is not a problem unless a INSERT .. ON DUPLICATE KEY statement is used where the resulting ON CONFLICT( "keyname" ) would be transformed to ON CONFLICT( "" ) which then fails.

To fix this, this now derives a key name in case it is omitted..

cc @costasovo

[brandonpayton]

This looks good, and the updated tests pass. Thanks, @akirk!