Fix header validity in MEI basic export. Fixes #3529 #1419

Job	Run time
Copy the font files	4s
Build ubuntu-20.04-g++-9	4m 19s
Build ubuntu-20.04-g++-10	4m 3s
Set up and cache emscripten	1m 51s
Build ubuntu-20.04-g++-11	5m 31s
Build ubuntu-20.04-clang-6.0	7m 32s
Build ubuntu-20.04-clang-9	4m 15s
Build ubuntu-20.04-clang-12	4m 20s
Build macos-latest-xcode-13.1	9m 22s
Build macos-latest-xcode-14.2	5m 33s
Build macos-11-g++-12	8m 4s
Build JS toolkit (nohumdrum, Building toolkit without humdrum, -c -H -M, verovio-toolkit.js*)	5m 24s
Build JS toolkit (light, Building toolkit without humdrum as light version, -c -H -l -M, verovio-...	5m 19s
Build JS toolkit (wasm, Building toolkit without humdrum as wasm, -c -H -w -M, veroviowasm, true)	6m 23s
Build JS toolkit (default, Building default toolkit with humdrum, -c -M, -hum.js)	6m 29s
Build documentation	7m 41s
Check settings for deployment	1s
Deploy JS toolkit	1m 12s
Deploy documentation	6m 40s
Skip deployment if set	0s
	1h 34m 3s

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