Otherwise the comma is treated as plain text by ParseContent.
$<STREQUAL:,> should be valid and true.
$<STREQUAL:,something> should be valid and false.
$<STREQUAL:,,> should be non-valid as it is 3 parameters.
$<STREQUAL:something,,> should be non-valid as it is 3 parameters.
Additionally, this allows reporting the correct error for other
expressions. For example $<TARGET_PROPERTY:,> should be invalid
because it has an empty target and empty property. It shouldn't
attempt to read the property ',' on the 'implicit this' target.
Stephen Kelly