documentclass[12pt]{article}

usepackage{tikz}
usepackage{etoolbox}

newcommand{specialsave}[1]{
    pgfkeys{
        /specialstore/place/.cd,
        a/.initial,
        c/.initial,#1,  % there cannot be a newline before #1
        c/.get=cvalA,  % this must come after #1, otherwise it won't get the values that are set
    }

    edefcvalB{pgfkeysvalueof{/specialstore/place/c}}
    defcompareval{pgfkeysnovalue}
    ifdefequal{cvalA}{compareval}{c is empty by comparison A}{c is non-empty by comparison A}

    ifdefequal{cvalB}{compareval}{c is empty by comparison B}{c is non-empty by comparison B}
}
begin{document}
    specialsave{
        a=the values of a,
    }
\

    specialsave{
        a=the values of a,
        c
    }
\

    specialsave{
        a=the values of a,
        c=
    }
\

    specialsave{
        a=the values of a,
        c={}
    }
end{document}

c is empty by comparison A
c is non-empty by comparison B

c is empty by comparison A
c is non-empty by comparison B

c is non-empty by comparison A
c is non-empty by comparison B

c is non-empty by comparison A
c is non-empty by comparison B