[lldb] Fix string summary of an empty NSPathStore2

Summary:
Printing a summary for an empty NSPathStore2 string currently prints random bytes behind the empty string pointer from memory (rdar://55575888).

It seems the reason for this is that the SourceSize parameter in the `ReadStringAndDumpToStreamOptions` - which is supposed to contain the string
length - actually uses the length 0 as a magic value for saying "read as much as possible from the buffer" which is clearly wrong for empty strings.

This patch adds another flag that indicates if we have know the string length or not and makes this behaviour dependent on that (which seemingly
was the original purpose of this magic value).

Reviewers: aprantl, JDevlieghere, shafik

Reviewed By: aprantl

Subscribers: christof, abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68010