Steps to reproduce:

1. Create an Item with Template for SSH Agent 
2. Key in Username 
3. Leave Password Empty
4. Write a SSH command 
5. Test it (Make sure your System accepts EmptyPassword for SSH)

Result:

Expected:
Expected to perform normal SSH command without failure

Details:
CentOS 8:
[root@]# ldd /usr/sbin/zabbix_server
linux-vdso.so.1 (0x00007ffeedb6a000)
libmariadb.so.3 => /lib64/libmariadb.so.3 (0x00007f640773c000)
libz.so.1 => /lib64/libz.so.1 (0x00007f6407525000)
libdl.so.2 => /lib64/libdl.so.2 (0x00007f6407321000)
libm.so.6 => /lib64/libm.so.6 (0x00007f6406f9f000)
libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f6406d7f000)
libssl.so.1.1 => /lib64/libssl.so.1.1 (0x00007f6406aeb000)
libcrypto.so.1.1 => /lib64/libcrypto.so.1.1 (0x00007f6406605000)
libxml2.so.2 => /lib64/libxml2.so.2 (0x00007f640629d000)
libodbc.so.2 => /lib64/libodbc.so.2 (0x00007f640602c000)
libnetsnmp.so.35 => /lib64/libnetsnmp.so.35 (0x00007f6405c83000)
libssh.so.4 => /lib64/libssh.so.4 (0x00007f6405a14000)
libOpenIPMI.so.0 => /lib64/libOpenIPMI.so.0 (0x00007f6405700000)
libOpenIPMIposix.so.0 => /lib64/libOpenIPMIposix.so.0 (0x00007f64054f8000)
libevent-2.1.so.6 => /lib64/libevent-2.1.so.6 (0x00007f640529f000)
libldap-2.4.so.2 => /lib64/libldap-2.4.so.2 (0x00007f6405052000)
liblber-2.4.so.2 => /lib64/liblber-2.4.so.2 (0x00007f6404e42000)
libcurl.so.4 => /lib64/libcurl.so.4 (0x00007f6404bb4000)
libresolv.so.2 => /lib64/libresolv.so.2 (0x00007f640499d000)
libpcre.so.1 => /lib64/libpcre.so.1 (0x00007f640472c000)
libc.so.6 => /lib64/libc.so.6 (0x00007f640436a000)
/lib64/ld-linux-x86-64.so.2 (0x00007f6407f6c000)
liblzma.so.5 => /lib64/liblzma.so.5 (0x00007f6404143000)
libltdl.so.7 => /lib64/libltdl.so.7 (0x00007f6403f39000)
librt.so.1 => /lib64/librt.so.1 (0x00007f6403d30000)
libgssapi_krb5.so.2 => /lib64/libgssapi_krb5.so.2 (0x00007f6403adb000)
libkrb5.so.3 => /lib64/libkrb5.so.3 (0x00007f64037f2000)
libk5crypto.so.3 => /lib64/libk5crypto.so.3 (0x00007f64035db000)
libcom_err.so.2 => /lib64/libcom_err.so.2 (0x00007f64033d7000)
libOpenIPMIutils.so.0 => /lib64/libOpenIPMIutils.so.0 (0x00007f64031cd000)
libgdbm.so.6 => /lib64/libgdbm.so.6 (0x00007f6402fbd000)
libsasl2.so.3 => /lib64/libsasl2.so.3 (0x00007f6402d9f000)
libnghttp2.so.14 => /lib64/libnghttp2.so.14 (0x00007f6402b78000)
libidn2.so.0 => /lib64/libidn2.so.0 (0x00007f640295a000)
libpsl.so.5 => /lib64/libpsl.so.5 (0x00007f6402749000)
libbrotlidec.so.1 => /lib64/libbrotlidec.so.1 (0x00007f640253d000)
libkrb5support.so.0 => /lib64/libkrb5support.so.0 (0x00007f640232c000)
libkeyutils.so.1 => /lib64/libkeyutils.so.1 (0x00007f6402128000)
libcrypt.so.1 => /lib64/libcrypt.so.1 (0x00007f6401eff000)
libunistring.so.2 => /lib64/libunistring.so.2 (0x00007f6401b7e000)
libbrotlicommon.so.1 => /lib64/libbrotlicommon.so.1 (0x00007f640195e000)
libselinux.so.1 => /lib64/libselinux.so.1 (0x00007f6401734000)
libpcre2-8.so.0 => /lib64/libpcre2-8.so.0 (0x00007f64014b0000)

[root@]# /usr/sbin/zabbix_server -V
zabbix_server (Zabbix) 5.0.3
Revision 146855bff3 24 August 2020, compilation time: Aug 24 2020 13:06:34

Copyright (C) 2020 Zabbix SIA
License GPLv2+: GNU GPL version 2 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it according to
the license. There is NO WARRANTY, to the extent permitted by law.

This product includes software developed by the OpenSSL Project
for use in the OpenSSL Toolkit (http://www.openssl.org/).

Compiled with OpenSSL 1.1.1c FIPS 28 May 2019
Running with OpenSSL 1.1.1g FIPS 21 Apr 2020

Wondering if this is implemented, if yes, it seems username shouldnt be a required field on Zabbix. (I maybe wrong)
https://api.libssh.org/master/group__libssh__auth.html#ga58e7c265236edbc97a2f117d3f23b4dd