Correct doc block return types.

[dereuromark]

Lets remove some "mixed" return types. Otherwise you could just as well omit it ;)
Also some more cleanup.

To stay BC I did not modify any code and rather fixed the doc block - even though in some places one should. It would be cleaner to always return string (empty string then instead of null).

E.g.

/**
 * Returns the schema name. Override this in subclasses.
 *
 * @return string|null schema name
 */
    public function getSchemaName() {
        return null;
    }

should be

/**
 * Returns the schema name. Override this in subclasses.
 *
 * @return string Schema name
 */
    public function getSchemaName() {
        return '';
    }

And stuff like

/**
 * Get the last error as a string.
 *
 * @return string|null Last error
 */
    public function lastError() {
        if (!empty($this->lastError)) {
            return $this->lastError['num'] . ': ' . $this->lastError['str'];
        }
        return null;
    }

should be

/**
 * Get the last error as a string.
 *
 * @return string Last error
 */
    public function lastError() {
        if (!empty($this->lastError)) {
            return $this->lastError['num'] . ': ' . $this->lastError['str'];
        }
        return '';
    }

Maybe for 2.6?

PS: Since quite a few of those "return null"s weren't documented, one could maybe even do this here as a bugfix instead of modifying the doc block?

[markstory]

Removing the null types and adding more consistent types makes sense for 2.6 if it does in the way you described.

[dereuromark]

Will do so after the 2.6 merge :) 2.6...master