Set default value 0 for compressorname

PiperOrigin-RevId: 525734520

testdata/src/test/assets/muxerdumps/mp4_with_float_metadata.mp4.dump

@@ -27,7 +27,7 @@ moov (772 bytes):
           Data = length 28, hash D535436B
         stbl (294 bytes):
           stsd (166 bytes):
-            Data = length 158, hash B2A35483
+            Data = length 158, hash 11532063
           stts (24 bytes):
             Data = length 16, hash E4FC6483
           stsz (24 bytes):

testdata/src/test/assets/muxerdumps/mp4_with_string_metadata.mp4.dump

@@ -27,7 +27,7 @@ moov (786 bytes):
           Data = length 28, hash D535436B
         stbl (294 bytes):
           stsd (166 bytes):
-            Data = length 158, hash B2A35483
+            Data = length 158, hash 11532063
           stts (24 bytes):
             Data = length 16, hash E4FC6483
           stsz (24 bytes):

testdata/src/test/assets/muxerdumps/mp4_with_xmp.mp4.dump

@@ -20,7 +20,7 @@ moov (658 bytes):
           Data = length 28, hash D535436B
         stbl (294 bytes):
           stsd (166 bytes):
-            Data = length 158, hash B2A35483
+            Data = length 158, hash 11532063
           stts (24 bytes):
             Data = length 16, hash E4FC6483
           stsz (24 bytes):

testdata/src/test/assets/muxerdumps/video_sample_entry_box_av1.dump

 av01 (125 bytes):
-  Data = length 117, hash CBDC1B53
+  Data = length 117, hash F51B373

testdata/src/test/assets/muxerdumps/video_sample_entry_box_h264.dump

 avc1 (150 bytes):
-  Data = length 142, hash 3B8029C9
+  Data = length 142, hash 9A2FF5A9

testdata/src/test/assets/muxerdumps/video_sample_entry_box_h265.dump

 hvc1 (214 bytes):
-  Data = length 206, hash 84B04E2E
+  Data = length 206, hash C7111A0E

testdata/src/test/assets/muxerdumps/video_sample_entry_box_h265_hdr10.dump

 hvc1 (240 bytes):
-  Data = length 232, hash C4EE9718
+  Data = length 232, hash 3E864AF8