baseparse: avoid bus error when no output frames

There are media with containers exposing streams but without no
output frames. In this case media processing will fail, even if
there are other streams that can be processed, in case of
playbin2, and fire an error in bus at EOS of all streams in
playbin3.

This patch demotes the error to a warning.
27 jobs for demote-error-baseparse in 65 minutes and 44 seconds (queued for 16 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #643459
gst indent

00:00:35

passed #643458
manifest

00:00:43

 
  Build
passed #643469
build cerbero cross-android universal

00:11:30

manual #643473
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #643470
build cerbero cross win32

00:09:13

passed #643471
build cerbero cross win64

00:08:17

passed #643468
build cerbero fedora x86_64

00:08:46

passed #643472
gst-macos-10.14
build cerbero macos x86_64

00:10:19

passed #643464
build clang fedora x86_64

00:06:57

passed #643460
build fedora x86_64

00:08:40

passed #643467
docker windows 1607
build msys2

00:12:07

passed #643461
build nodebug fedora x86_64

00:07:21

passed #643462
build static fedora x86_64

00:15:17

passed #643463
build static nodebug fedora x86_64

00:03:36

passed #643465
docker windows 1607
build vs2017 amd64

00:11:07

passed #643466
docker windows 1607
build vs2017 x86

00:11:22

 
  Test
passed #643474
check fedora

00:05:09

passed #643475
integration testsuites fedora

00:13:12

passed #643480
valgrind bad

00:09:39

passed #643477
valgrind base

00:18:53

passed #643476
valgrind core

00:12:19

passed #643481
valgrind ges

00:10:59

passed #643478
valgrind good

00:11:18

passed #643479
valgrind ugly

00:02:36

 
  Integrate
passed #643482
cross-android universal examples

00:08:29

manual #643484
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #643483
documentation

00:05:23