gralloc4-vts: correctly instantiate std::optional

std::optional defaults to containing std::nullopt_t. Using "->" or
"." on an std::optional object that does not contain a value is
undefined. Clang seems to ignore the whole line.

SetSmpte2086 and SetCta861_3 both incorrectly instantiate their
std::optional types. The std::optional types contain std::nullopt_t
when they are passed into encode. When mapper decodes the fields,
they are empty.

Update std::optionals to be instantiated correctly.

Bug: 149931805
Test: VtsHalGraphicsMapperV4_0

Change-Id: I046242f8261a5378228231b89b2eedec242667cb