First of all JavadocUtil#parseJavadocVersion(String) must be improved according to the specs of JEP 223
Next AbstractJavadocMojo#setFJavadocVersion(File) contains a fallback based on SystemUtils.JAVA_VERSION_FLOAT from commons-lang, either rewrite it ourselves or see how commons-lang will solve this.
[INFO] --- maven-javadoc-plugin:2.10.3:javadoc (default-cli) @ version ---
[WARNING] Unable to find the javadoc version: Unrecognized version of Javadoc: 'java version "9-ea"
Java(TM) SE Runtime Environment (build 9-ea+113)
Java HotSpot(TM) 64-Bit Server VM (build 9-ea+113, mixed mode)
' near index 37
[WARNING] Using the Java the version instead of, i.e. 0.0
both initial parsing and fallback version are wrong...