Allow config files to specify no bootnodes

pantheon/src/main/java/tech/pegasys/pantheon/cli/PantheonCommand.java

@@ -188,7 +188,10 @@ protected KeyLoader getKeyLoader() {
   void setBootnodes(final List<String> values) {
     try {
       bootNodes =
-          values.stream().map((s) -> EnodeURL.fromString(s).toURI()).collect(Collectors.toList());
+          values.stream()
+              .filter(value -> !value.isEmpty())
+              .map(value -> EnodeURL.fromString(value).toURI())
+              .collect(Collectors.toList());
     } catch (final IllegalArgumentException e) {
       throw new ParameterException(commandLine, e.getMessage());
     }

pantheon/src/test/java/tech/pegasys/pantheon/cli/PantheonCommandTest.java

@@ -234,6 +234,22 @@ public void callingWithConfigOptionButInvalidContentTomlFileShouldDisplayHelp()
     assertThat(commandOutput.toString()).isEmpty();
   }
 
+  @Test
+  public void callingWithNoBootnodesConfig() throws Exception {
+    assumeTrue(isFullInstantiation());
+
+    final URL configFile = this.getClass().getResource("/no_bootnodes.toml");
+    final Path toml = createTempFile("toml", Resources.toString(configFile, UTF_8));
+
+    parseCommand("--config-file", toml.toAbsolutePath().toString());
+
+    verify(mockRunnerBuilder).ethNetworkConfig(ethNetworkConfigArgumentCaptor.capture());
+    assertThat(ethNetworkConfigArgumentCaptor.getValue().getBootNodes()).isEmpty();
+
+    assertThat(commandErrorOutput.toString()).isEmpty();
+    assertThat(commandOutput.toString()).isEmpty();
+  }
+
   @Test
   public void callingWithConfigOptionButInvalidValueTomlFileShouldDisplayHelp() throws Exception {
     assumeTrue(isFullInstantiation());

pantheon/src/test/resources/no_bootnodes.toml

@@ -0,0 +1 @@
+bootnodes=[]