The virtual method named `get_caps` in both `GstBaseSrc` and
`GstBaseSink` has a `filter` parameter which can be `NULL` (the
default implementation in GstBaseSrc already considers the case).
Before this commit, there was no gtk-doc annotation representing this
fact, which caused the corresponding entry in the GIR file to also
miss this fact.

This caused bugs in other places, such inducing the Vala compiler to
introduce a wrongly assert on `(filter != NULL)` in every
implementation of the `get_caps` method implemented in Vala.