TYPO3 - Return title & sitename if the browsertitle (tx_metaseo_pagetitle_rel) is not set

Question

How can I use the title & sitename if the Browsertitle (tx_metaseo_pagetitle_rel) is not set?

The below always returns the sitename as well ...

page.headerData {
    5 = TEXT
    5 { 
        field = tx_metaseo_pagetitle_rel
        ifEmpty.field = title
        noTrimWrap = |<title>| - |
    }
    10 = TEXT
    10.data = GLOBAL:TYPO3_CONF_VARS|SYS|sitename
    20 = TEXT
    20.value = </title>         
}

Answer 1

in TYPO3 you have multiple options to use a field and, in case of empty value, use another.
And you should avoid splitting tags to different objects.

my attempt for your example would be:

page.headerData {
    10 = COA
    10 {
        wrap = <title>|</title>

        10 = TEXT
        10.field = tx_metaseo_pagetitle_rel // title

        20 = TEXT
        20.data = GLOBAL:TYPO3_CONF_VARS|SYS|sitename
        20.noTrimWrap = | - ||
    }
}

rereading your question I came up with the the idea:
you want either only the value of the field tx_metaseo_pagetitle_rel or the page title with the sitename attached.
That would result in a different typoscript:

page.headerData {
    10 = TEXT
    10 {
        wrap = <title>|</title>

        field = tx_metaseo_pagetitle_rel
        ifEmpty.cObject = TEXT
        ifEmpty.cObject {
            value = {field:title} - {GLOBAL:TYPO3_CONF_VARS|SYS|sitename}
            insertData = 1
        }
    }
}