SPIFFS Class Dir no have bool operator

[kristophert]

Class Dir in FS.cpp no have bool operator.
In reference.md is written:

openDir
SPIFFS.openDir(path)
Opens a directory given its absolute path. Returns a Dir object. 
To check if directory was opened successfully, use the boolean operator, similar to opening a file

In class File is ok:

File::operator bool() const {
    return !!_p;
}