Bug#50507 Behavior of logarithmic functions depends on ERROR_FOR_DIVISION_BY_ZERO

The manual says that if the argument of LN(), LOG2(), LOG10() or LOG()
functions is less than or equal to 0, then NULL is returned.

However, if the ERROR_FOR_DIVISION_BY_ZERO server SQL mode is enabled, those
functions return the "division by zero" error when in strict mode (or a
warning otherwise) instead of NULL. This is counter-intuitive and not
documented.

Fix: introduce signal_invalid_argument_for_log() which reports
"Invalid argument for logarithm" rather than "Division by 0"

Bug#50507 asked for Returning NULL regardless of the SQL mode.
This patch returns an error if strict mode is enabled.
This is in line with WL#7467 which will remove ERROR_FOR_DIVISION_BY_ZERO
and make it part of strict mode.