In pgtk, child-frame-border-width is set to 0 by default, and it cannot
be set to nil.
According to the manual it should be possible for child-frame-border-
width to be nil. In such cases, the value of internal-border-width is
used as the border width (see 30.4.3.4 "Layout Parameters"; and
function FRAME_INTERNAL_BORDER_WIDTH in frame.h).

The problem causes missing child-frame borders for code that depends on
this behavior (e.g., lsp-doc-ui in the lsp-ui package), and it will
cause errors for code that tries to set child-frame-border-width to
nil.

The attached patch fixes this. It essentially replicates the respective
behavior of xfns.c.