Fix (GraphQL): raise exception when blob is missing

Failures:

  1) API::V2::GraphqlController when authenticated mutations dossierEnvoyerMessage upload error should fail
     Failure/Error: expect(gql_errors).to eq(nil)

       expected: nil
            got: [{:backtrace=>["/usr/local/bundle/ruby/2.7.0/gems/graphql-1.12.5/lib/graphql/backtrace/tracer.rb:64:i...]

app/graphql/mutations/base_mutation.rb

@@ -5,6 +5,8 @@ module Mutations
     def validate_blob(blob_id)
       begin
         blob = ActiveStorage::Blob.find_signed(blob_id)
+        raise ActiveSupport::MessageVerifier::InvalidSignature if blob.nil?
+
         # open downloads the file and checks its hash
         blob.open { |f| }
         true