Ignore non property structures to allow more advanced string building

[mzeinstra]

I would like to create a structure with name (P9) and birth (P19) and death dates (P21):

P9 (P19 - P21)
But if there is no P19 I want
P9 ( - P21)
and if there is no P21 I want
P9 (P19 - )

Currently I cannot configure this. It would be helpful to allow for 'non property paramaters' in the config.

                "*": {
                    "P9": "$",
                    "P9.P11": " $",
                    "P9.P12": ", $",
                    "P19": " ($",
                    "P21": " - $)"
                }

Ideally I would want to do something like:

                "*": {
                    "P9": "$",
                    "P9.P11": " $",
                    "P9.P12": ", $",
                    "_1": " (",
                    "P19": "$",
                    "_2": " - ",
                    "P21": "$",
                    "_3": ")"
                }

I tried using other Properties, but it appears that when no $ is found, not string is placed. I.e. this hack does not work:

                "*": {
                    "P9": "$",
                    "P9.P11": " $",
                    "P9.P12": ", $",
                    "P23": " (",
                    "P19": "$",
                    "P36": " - ",
                    "P21": "$",
                    "P71": ")"
                }

Where P23, P36, and P71 are required fields in the item, so we expect them to be there. But without a $ the line is ignored.