[MJAVADOC-810] [REGRESSION] MJAVADOC-791 causes forked Maven execution fail if any toolchains or settings file isn't present

As request carries the "effective" value (default or user overriden
ones) but it does not mean they exist. And as invoker used,
effect is same as user would use -s/-t CLI commands with non-existent
files: mvn CLI failure.

Co-authored-by: Michael Osipov <michaelo@apache.org>

This closes #313

Author: cstamas

src/main/java/org/apache/maven/plugins/javadoc/JavadocUtil.java

@@ -780,10 +780,18 @@ protected static void invokeMaven(
         InvocationRequest request = new DefaultInvocationRequest();
         request.setBaseDirectory(projectFile.getParentFile());
         request.setPomFile(projectFile);
-        request.setGlobalSettingsFile(globalSettingsFile);
-        request.setUserSettingsFile(userSettingsFile);
-        request.setGlobalToolchainsFile(globalToolchainsFile);
-        request.setToolchainsFile(userToolchainsFile);
+        if (globalSettingsFile != null && globalSettingsFile.isFile()) {
+            request.setGlobalSettingsFile(globalSettingsFile);
+        }
+        if (userSettingsFile != null && userSettingsFile.isFile()) {
+            request.setUserSettingsFile(userSettingsFile);
+        }
+        if (globalToolchainsFile != null && globalToolchainsFile.isFile()) {
+            request.setGlobalToolchainsFile(globalToolchainsFile);
+        }
+        if (userToolchainsFile != null && userToolchainsFile.isFile()) {
+            request.setToolchainsFile(userToolchainsFile);
+        }
         request.setBatchMode(true);
         if (log != null) {
             request.setDebug(log.isDebugEnabled());