fix #7173: write config: drop empty tag list

When the last tag of a VM gets removed, for example in the web UI, the
API gets an empty 'tags' value, which then ends up as an empty 'tags:'
line in the config and in the output of 'qm config'. As the config
parser reads that line back as a single space, it also gets written out
again on every following config change.

Skip a tag list without any tag when writing the config, which also
cleans up existing configs with such a line on their next change. The
pending section still needs to keep it, otherwise the removal of all
tags would get lost between writing the pending change and applying it.

Signed-off-by: Michal Fox <me@dualfroz.com>

src/PVE/QemuServer.pm

@@ -2167,6 +2167,8 @@ sub write_vm_config {
 
         foreach my $key (sort keys %$conf) {
             next if $key =~ /^(digest|description|pending|snapshots|special-sections)$/;
+            # an empty tag list is only needed as pending change for removing all tags
+            next if $key eq 'tags' && !$pending && $conf->{$key} =~ m/^\s*$/;
             $raw .= "$key: $conf->{$key}\n";
         }
         return $raw;

src/test/parse-config-expected/empty-tags.conf

@@ -0,0 +1,8 @@
+# empty tag list
+boot: order=scsi0
+cores: 1
+memory: 512
+name: notags
+ostype: l26
+scsi0: local-lvm:vm-100-disk-0,size=4G
+scsihw: virtio-scsi-single

src/test/parse-config-input/empty-tags.conf

@@ -0,0 +1,9 @@
+# empty tag list
+boot: order=scsi0
+cores: 1
+memory: 512
+name: notags
+ostype: l26
+scsi0: local-lvm:vm-100-disk-0,size=4G
+scsihw: virtio-scsi-single
+tags: 

src/test/run_parse_config_tests.pl

@@ -26,7 +26,7 @@ my $OUTPUT_DIR = './parse-config-output';
 my $EXPECTED_DIR = './parse-config-expected';
 
 # NOTE update when you add/remove tests
-plan tests => 2 * 10;
+plan tests => 2 * 11;
 
 sub run_tests {
     my ($strict) = @_;