journal: _form: only show image if avatar is set
journal: _form: only show image if avatar is set

Interestingly, for the '#new' method, '@journal' is set to
'Journal.new', and in rails console, the #avatar field is set to nil.
However, in actual practice, it is set to an empty string. This is why
we call the '#blank?' method instead of '#nil?'.

listx/pf,listx/pf