Apache less than 2.4.7 does not have the setIfEmpty verb. Looking up online I found a few SetEnv attempts of replicating setting a header variable if not already set that went along the lines of
SetEnvIf X-My-Header "" no_my_header RequestHeader set X-My-Header "value" env=no_my_header
I've managed to get it working in a one liner
Header set Content-Security-Policy "..." "expr=-z resp('Content-Security-Policy')"
expr- the following is an expression to evaluate
-z- return true if the following string is empty
resp- function for looking up response header values
Content-Security-Policy- the header in question
Nice way to preserve pre-set headers based on .htaccess inheritance. In this case, I'm adding
data: to my Content-Security-Policy specifically for a pie-chart SVG to be able to use inline SVGs for patterns. So I can keep my fine-grained control.