Any non empty string is expected to be larger than the empty string. Contrary to this, SELECT ... WHERE textfield > '' fails where the text data begins with a control character like newline or tab.
This affects multiple collations (tested with utf8 and latin1 standards), only binary comparison works as expected.
Reproduce / test:
Any control character at the beginning of the text triggers this behaviour.
Workaround: use BINARY or *_bin collations or compare textfield <> ''.