How to display an empty string in outreg2

[Luck Good]

I learned yesterday that I can use empty string as a placeholder to try 2 specifications: one with a variable and the other without. See the solution here. I have an additional question regarding how to save such different specifications using outreg2.

webuse lifeexp
local p12 popgrowth ""

foreach p in `p12'{

    reg lexp gnppc `p'
    outreg2 using "test.xml", addtext(popgrowth, `popgrowth')

}

The output in test.xml is not property formatted due to the empty string of 2nd iteration in macro p.

I understand that it is trivial for this particular example since the output implies which specification is used. However, I want to learn the proper way to be able to display this empty string.

Through some trial and error, I found the problem is because the macro p becomes undefined in the 2nd iteration.

webuse lifeexp
local p12 popgrowth ""

foreach p in `p12'{

    reg lexp gnppc `p'
    outreg2 using "test.xml", addtext(popgrowth, `popgrowth')
    macro list _p

}