Prevent misleading warning being printed to the lighttpd error log when importing from teleporter backup

[PromoFaux]

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)

---

What does this PR aim to accomplish?:

Fixes #2117 by adding a check for table name before attempting to flush it

[yubiuser]

As I've been asked:
Rather then specify exactly which tables to ignore, skip the flush if the table does not exists.
https://stackoverflow.com/questions/1601151/how-do-i-check-in-sqlite-whether-a-table-exists

[PromoFaux]

FP'd new code, tested by using adding some error logging into it as below:

// Flush table if requested, only flush each table once
if($flush && !in_array($table, $flushed_tables))
{
	$tableExists = $db->querySingle("SELECT name FROM sqlite_master WHERE type='table' AND name='".$table."';");
	if ($tableExists)
	{
		error_log("doing ".$table);
		$db->exec("DELETE FROM \"".$table."\"");
		array_push($flushed_tables, $table);
	}
	else
	{
		error_log("not doing ".$table);
	}
}

Which outputs:

2022-02-12 13:31:15: mod_fastcgi.c.487) FastCGI-stderr:doing adlist
2022-02-12 13:31:15: mod_fastcgi.c.487) FastCGI-stderr:doing adlist_by_group
2022-02-12 13:31:15: mod_fastcgi.c.487) FastCGI-stderr:not doing blacklist
2022-02-12 13:31:15: mod_fastcgi.c.487) FastCGI-stderr:not doing regex_blacklist
2022-02-12 13:31:15: mod_fastcgi.c.487) FastCGI-stderr:doing client
2022-02-12 13:31:15: mod_fastcgi.c.487) FastCGI-stderr:doing client_by_group
2022-02-12 13:31:15: mod_fastcgi.c.487) FastCGI-stderr:doing domain_audit
2022-02-12 13:31:15: mod_fastcgi.c.487) FastCGI-stderr:doing domainlist_by_group
2022-02-12 13:31:15: mod_fastcgi.c.487) FastCGI-stderr:doing group
2022-02-12 13:31:15: mod_fastcgi.c.487) FastCGI-stderr:not doing whitelist
2022-02-12 13:31:15: mod_fastcgi.c.487) FastCGI-stderr:not doing regex_whitelist

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-14-web-v5-11-and-core-v5-9-released/53529/1