It's not entirely clear from the grammar

[177]   	StringConstructor	   ::=   	"``[" StringConstructorContent "]``"

[178]   	StringConstructorContent	   ::=   	StringConstructorChars (StringConstructorInterpolation StringConstructorChars)*

[179]   	StringConstructorChars	   ::=   	(Char* - (Char* ('`{' | ']``') Char*))  /* ws: explicit */

whether the whitespace in 

``[ ]``

``[ hello ]``

``[ `{ $x }` ]``

forms part of the constructed string, since the ws: explicit rule isn't applying to production [177].

[Definition: Whitespace and Comments function as symbol separators. For the most part, they are not mentioned in the grammar, and may occur between any two terminal symbols mentioned in the grammar, except where that is forbidden by the /* ws: explicit */ annotation in the EBNF, or by the /* xgc: xml-version */ annotation.]

If I'm not mistaken, whitespace or comments may appear between the terminal symbols "``[" and Char.