Skip nil IP addresses in ipv6 fix

Fixes GH-6558

plugins/providers/virtualbox/action/network_fix_ipv6.rb

@@ -27,7 +27,8 @@ module VagrantPlugins
           env[:machine].config.vm.networks.each do |type, options|
             next if type != :private_network
             options = scoped_hash_override(options, :virtualbox)
-            next if options[:ip] == ""
+            next if options[:ip].to_s.strip == ""
+
             if IPAddr.new(options[:ip]).ipv6?
               has_v6 = true
               break

test/unit/plugins/providers/virtualbox/action/network_fix_ipv6_test.rb

@@ -0,0 +1,33 @@
+require_relative "../base"
+
+describe VagrantPlugins::ProviderVirtualBox::Action::NetworkFixIPv6 do
+  include_context "unit"
+  include_context "virtualbox"
+
+  let(:iso_env) do
+    env = isolated_environment
+    env.vagrantfile("")
+    env.create_vagrant_env
+  end
+
+  let(:machine) do
+    iso_env.machine(iso_env.machine_names[0], :dummy)
+  end
+
+  let(:env) {{ machine: machine }}
+  let(:app) { lambda { |*args| }}
+
+  subject { described_class.new(app, env) }
+
+  it "ignores nil IP addresses" do
+    allow(machine.config.vm).to receive(:networks)
+      .and_return(private_network: { ip: nil })
+    expect { subject.call(env) }.to_not raise_error
+  end
+
+  it "blank nil IP addresses" do
+    allow(machine.config.vm).to receive(:networks)
+      .and_return(private_network: { ip: "" })
+    expect { subject.call(env) }.to_not raise_error
+  end
+end