Raise ArgumentError in push.rb if job is nil

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

lib/prometheus/client/push.rb

@@ -22,6 +22,10 @@ class Push
       attr_reader :job, :instance, :gateway, :path
 
       def initialize(job, instance = nil, gateway = nil)
+        unless job
+          raise ArgumentError, "job cannot be nil"
+        end
+
         @mutex = Mutex.new
         @job = job
         @instance = instance

spec/prometheus/client/push_spec.rb

@@ -24,6 +24,12 @@
       expect(push.gateway).to eql('http://pu.sh:1234')
     end
 
+    it 'raises an ArgumentError if the job is not provided' do
+      expect do
+        Prometheus::Client::Push.new(nil)
+      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