Scriptblock with IF statement as parameter of function

Question

I have this script file contains:

   function run_command 
{
    param    
    (
        [string] $scriptblock
    )

    $sb = [Scriptblock]::Create($scriptblock)
    Start-Job -ScriptBlock $sb | Wait-Job | Receive-Job
}

$a = "aaa"

$scriptblock = @"
if($a){echo $a}
else {echo nene}
"@

run_command -scriptblock $scriptblock

I have two problem: 1. If the scriptblock contains IF statement, the execution failed with error like "The term 'someofArguments' is not recognized as the name of a cmdlet...".

But without IF statement everything is working fine. So how to send that scriptblock with IF statement?

2. How to avoid using backtick in $command with variables $true, $false and mainly $null? Because without backtick they are translated to true,false and nothing.

Thank you.

PS: I found this topics PowerShell function with parameterized script block and similar posts, that are saying to send scriptblock as string and in function itself convert it to type scriptblock.

EDIT: I figure out this solution, but it is not very nice. Do someone know easier solution than put backtick like this?

$scriptblock = @"
`$a=`"$a`"
if(`$a){echo `$a}
else {echo nene}
"@