Chaining methods using Symbol#to_proc shorthand in Ruby?

No, there's no shorthand for that. You could define a method:

def really_empty?(x)
  x.strip.empty?
end

and use method:

array.reject(&method(:really_empty?))

or use a lambda:

really_empty = ->(x) { x.strip.empty? }
array.reject(&really_empty)

but I wouldn't call either of those better unless you have a use for really_empty? in enough places that splitting up the logic makes sense.

However, since you're using Rails, you could just use blank? instead of .strip.empty?:

array.reject(&:blank?)

Note that nil.blank? is true whereas nil.strip.empty? just hands you an exception so they're not quite equivalent; however, you probably want to reject nils as well so using blank? might be better anyway. blank? also returns true for false, {}, and [] but you probably don't have those in your array of strings.

It would be quite nice to write the above like that but it's not a supported syntax, the way that you would want to chain methods using the to_proc syntax (&:) is like so:

.map(&:strip).reject(&:empty?)