Force config value type and add test coverage

When extracting the config value from the data, force it to an Array
type for the size check. Include a test case that includes missing
configuration information to verify it does not produce an error.

plugins/providers/docker/driver.rb

@@ -348,9 +348,8 @@ module VagrantPlugins
 
         network_info = inspect_network(all_networks)
         network_info.each do |network|
-          config = network["IPAM"]["Config"]
+          config = Array(network["IPAM"]["Config"])
-          if (defined?(config.size) &&
+          if (config.size > 0 &&
-            config.size > 0 &&
             config.first["Subnet"] == subnet_string)
             @logger.debug("Found existing network #{network["Name"]} already configured with #{subnet_string}")
             return network["Name"]

test/unit/plugins/providers/docker/driver_test.rb

@@ -667,21 +667,68 @@ describe VagrantPlugins::DockerProvider::Driver do
     let(:subnet_string) { "172.20.0.0/16" }
     let(:network_names) { ["vagrant_network_172.20.0.0/16", "bridge", "null" ] }
 
-    it "returns network name if defined" do
+    before do
       allow(subject).to receive(:list_network_names).and_return(network_names)
       allow(subject).to receive(:inspect_network).and_return(JSON.load(docker_network_struct))
+    end
 
+    it "returns network name if defined" do
       network_name = subject.network_defined?(subnet_string)
       expect(network_name).to eq("vagrant_network_172.20.0.0/16")
     end
 
     it "returns nil name if not defined" do
-      allow(subject).to receive(:list_network_names).and_return(network_names)
-      allow(subject).to receive(:inspect_network).and_return(JSON.load(docker_network_struct))
-
       network_name = subject.network_defined?("120.20.0.0/24")
       expect(network_name).to eq(nil)
     end
+
+    context "when config information is missing" do
+      let(:docker_network_struct) do
+        [
+          {
+            "Name": "bridge",
+            "Id": "ae74f6cc18bbcde86326937797070b814cc71bfc4a6d8e3e8cf3b2cc5c7f4a7d",
+            "Created": "2019-03-20T14:10:06.313314662-07:00",
+            "Scope": "local",
+            "Driver": "bridge",
+            "EnableIPv6": false,
+            "IPAM": {
+              "Driver": "default",
+              "Options": nil,
+            },
+            "Internal": false,
+            "Attachable": false,
+            "Ingress": false,
+            "ConfigFrom": {
+              "Network": ""
+            },
+            "ConfigOnly": false,
+            "Containers": {
+              "a1ee9b12bcea8268495b1f43e8d1285df1925b7174a695075f6140adb9415d87": {
+                "Name": "vagrant-sandbox_docker-1_1553116237",
+                "EndpointID": "fc1b0ed6e4f700cf88bb26a98a0722655191542e90df3e3492461f4d1f3c0cae",
+                "MacAddress": "02:42:ac:11:00:02",
+                "IPv4Address": "172.17.0.2/16",
+                "IPv6Address": ""
+              },
+              "Options": {
+                "com.docker.network.bridge.default_bridge": "true",
+                "com.docker.network.bridge.enable_icc": "true",
+                "com.docker.network.bridge.enable_ip_masquerade": "true",
+                "com.docker.network.bridge.host_binding_ipv4": "0.0.0.0",
+                "com.docker.network.bridge.name": "docker0",
+                "com.docker.network.driver.mtu": "1500"
+              },
+              "Labels": {}
+            },
+          }
+        ].to_json
+      end
+
+      it "should not raise an error" do
+        expect { subject.network_defined?(subnet_string) }.not_to raise_error
+      end
+    end
   end
 
   describe '#network_containing_address' do