Raise ArgumentError in push.rb if job is empty

Signed-off-by: Chris Sinjakli <chris@sinjakli.co.uk>

lib/prometheus/client/push.rb

@@ -23,6 +23,7 @@ class Push
 
       def initialize(job:, instance: nil, gateway: DEFAULT_GATEWAY)
         raise ArgumentError, "job cannot be nil" if job.nil?
+        raise ArgumentError, "job cannot be empty" if job.empty?
 
         @mutex = Mutex.new
         @job = job

spec/prometheus/client/push_spec.rb

@@ -30,6 +30,12 @@
       end.to raise_error ArgumentError
     end
 
+    it 'raises an ArgumentError if the job is empty' do
+      expect do
+        Prometheus::Client::Push.new(job: "")
+      end.to raise_error ArgumentError
+    end
+
     it 'raises an ArgumentError if the given gateway URL is invalid' do
       ['inva.lid:1233', 'http://[invalid]'].each do |url|
         expect do