Don't pass default GLib marshallers for signals

By passing `NULL` to `g_signal_new` instead of a marshaller, GLib will
actually internally optimize the signal (if the marshaller is available
in GLib itself) by also setting the valist marshaller. This makes the
signal emission a bit more performant than the regular marshalling,
which still needs to box into `GValue` and call libffi in case of a
generic marshaller.

Note that for custom marshallers, one would use
`g_signal_set_va_marshaller()` with the valist marshaller instead.
24 jobs for wip/nielsdg/generic-marshallers-use-NULL in 51 minutes and 3 seconds (queued for 10 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #589398
gst indent

00:01:06

passed #589397
manifest

00:00:50

 
  Build
passed #589408
build cerbero android universal

00:14:16

passed #589409
build cerbero cross win32

00:12:13

passed #589410
build cerbero cross win64

00:08:18

passed #589407
build cerbero fedora x86_64

00:06:27

manual #589412
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #589411
gst-macos-10.14
build cerbero macos x86_64

00:11:04

passed #589403
build clang fedora x86_64

00:08:51

passed #589399
build fedora x86_64

00:06:26

passed #589406
docker windows 1607
build msys2

00:15:32

passed #589400
build nodebug fedora x86_64

00:04:54

passed #589401
build static fedora x86_64

00:13:32

passed #589402
build static nodebug fedora x86_64

00:05:09

passed #589404
docker windows 1607
build vs2017 amd64

00:11:23

passed #589405
docker windows 1607
build vs2017 x86

00:10:47

 
  Test
failed #589413
check fedora

00:05:11

passed #589414
integration testsuites fedora

00:21:05

passed #589416
valgrind bad

00:07:05

passed #589417
valgrind ges

00:07:52

passed #589415
valgrind good

00:13:43

 
  Integrate
passed #589418
android universal examples

00:09:19

passed #589419
documentation

00:05:08

skipped #589420
gst-ios-12.2 allowed to fail manual
ios universal examples
 
Name Stage Failure
failed
check fedora Test
           Total: 3134
Uploading artifacts...
gst-build/build/meson-logs/: found 2 matching files
validate-logs: found 3553 matching files

Uploading artifacts to coordinator... ok
id=589413 responseStatus=201 Created token=MhpRBPcK
Uploading artifacts...
validate-logs/*.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=589413 responseStatus=201 Created token=MhpRBPcK
ERROR: Job failed: exit code 1