audio,video,gl,webrtc: Mark manually-confirmed to_string fns as non-NULL

These functions have been checked to never return NULL. The overrides
can hopefully be gone again when C contains better annotations and
trust_return_value_nullability is enabled.

gstreamer-audio/Gir.toml

@@ -134,6 +134,11 @@ status = "generate"
     # Platform dependant
     manual = true
 
+    [[object.function]]
+    name = "to_string"
+        [object.function.return]
+        nullable = false
+
 [[object]]
 name = "GstAudio.AudioStreamAlign"
 status = "generate"

gstreamer-gl/Gir.toml

@@ -426,6 +426,11 @@ status = "generate"
     name = "any"
     ignore = true
 
+    [[object.function]]
+    name = "to_string"
+        [object.function.return]
+        nullable = false
+
 [[object]]
 name = "GstGL.GLPlatform"
 status = "generate"
@@ -436,6 +441,11 @@ status = "generate"
     name = "any"
     ignore = true
 
+    [[object.function]]
+    name = "to_string"
+        [object.function.return]
+        nullable = false
+
 [[object]]
 name = "GstGL.GLSLProfile"
 status = "generate"

gstreamer-video/Gir.toml

@@ -21,7 +21,6 @@ generate = [
     "GstVideo.VideoFormatFlags",
     "GstVideo.VideoColorMatrix",
     "GstVideo.VideoMultiviewMode",
-    "GstVideo.VideoFieldOrder",
     "GstVideo.VideoMultiviewFramePacking",
     "GstVideo.VideoFilter",
     "GstVideo.VideoCaptionType",
@@ -389,6 +388,11 @@ status = "generate"
     name = "nv12_32l32"
     version = "1.18"
 
+    [[object.function]]
+    name = "to_string"
+        [object.function.return]
+        nullable = false
+
 [[object]]
 name = "GstVideo.VideoSink"
 status = "generate"
@@ -451,6 +455,11 @@ status = "generate"
     name = "alternate"
     version = "1.16"
 
+    [[object.function]]
+    name = "to_string"
+        [object.function.return]
+        nullable = false
+
 [[object]]
 name = "GstVideo.VideoChromaSite"
 status = "generate"
@@ -458,6 +467,11 @@ status = "generate"
     name = "unknown"
     ignore = true
 
+    [[object.function]]
+    name = "to_string"
+        [object.function.return]
+        nullable = false
+
 [[object]]
 name = "GstVideo.VideoFlags"
 status = "generate"
@@ -524,3 +538,12 @@ status = "generate"
     [[object.member]]
     name = "linear"
     version = "1.18"
+
+[[object]]
+name = "GstVideo.VideoFieldOrder"
+status = "generate"
+
+    [[object.function]]
+    name = "to_string"
+        [object.function.return]
+        nullable = false

gstreamer-webrtc/Gir.toml

@@ -22,7 +22,6 @@ generate = [
     "GstWebRTC.WebRTCICEConnectionState",
     "GstWebRTC.WebRTCICERole",
     "GstWebRTC.WebRTCICEComponent",
-    "GstWebRTC.WebRTCSDPType",
     "GstWebRTC.WebRTCDTLSSetup",
     "GstWebRTC.WebRTCPeerConnectionState",
     "GstWebRTC.WebRTCRTPTransceiverDirection",
@@ -88,3 +87,12 @@ final_type = true
     name = "on_error"
     # takes ownership of SDP message
     manual = true
+
+[[object]]
+name = "GstWebRTC.WebRTCSDPType"
+status = "generate"
+
+    [[object.function]]
+    name = "to_string"
+        [object.function.return]
+        nullable = false