The help for ?Startup described the mechanism for doing this (in the Details section):
Lines in a site or user environment file should be either comment lines starting with #, or lines of the form name=value. The latter sets the environmental variable name to value, overriding an existing value. If value contains an expression of the form ${foo-bar}, the value is that of the environmental variable foo if that is set, otherwise bar. For ${foo:-bar}, the value is that of foo if that is set to a non-empty value, otherwise bar. (If it is of the form ${foo}, the default is "".) This construction can be nested, so bar can be of the same form (as in ${foo-${bar-blah}}). Note that the braces are essential: for example $HOME will not be interpreted.
I haven't tested this, but IIUC, this means you can use: