[MJAVADOC-736] Plugin fails to detect module descriptor if mvn clean run during initialize phase

**[Daniel Widdis](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=dbwiddis)** opened **[MJAVADOC-736](https://issues.apache.org/jira/browse/MJAVADOC-736?redirect=false)** and commented

When the clean goal of maven-clean-plugin is invoked during the initialize phase, maven-javadoc-plugin fails to find the module descriptor. Analysis of the search logic reveals that if an output directory exists, but either a jar doesn't exist or the jar does not contain an Automatic-Module-Name, the mojo never reaches the code branch where a module descriptor is searched for.

Steps to reproduce:
1. Create a simple project with a module-info.java file, but no automatic module name.
2. Invoke the `clean` goal of `maven-clean-plugin` during the initialize phase (either automatically in pom, or by running `mvn clean`)
3. Run `mvn javadoc:javadoc` or `mvn javadoc:jar`

Expected result:

`[INFO] — maven-javadoc-plugin:3.4.1:javadoc (default-cli) @ foo ---`
`[INFO] Configuration changed, re-generating javadoc.`
`[INFO] ------------------------------------------------------------------------`
`[INFO] BUILD SUCCESS`

Actual result:

`[INFO] — maven-javadoc-plugin:3.4.1:javadoc (default-cli) @ foo ---`
`[ERROR] Creating an aggregated report for both named and unnamed modules is not possible.`
`[ERROR] Ensure that every module has a module descriptor or is a jar with a MANIFEST.MF containing an Automatic-Module-Name.`
`[ERROR] Fix the following projects:`
`[ERROR]  - foo:foo`

Minimal reproducer project: https://github.com/dbwiddis/foo

Probable source of the error: https://github.com/apache/maven-javadoc-plugin/blob/master/src/main/java/org/apache/maven/plugins/javadoc/AbstractJavadocMojo.java#L4999-L5096

In AbstractJavadocMojo beginning at line 4999, a list of unnamedProjects is created. It is added to on line 5070 when `result` shows a failure to find the module descriptor. 

There are two chances to set result, one at line 5014 (searching for automatic module name in the jar) and one at line 5037 (searching for module descriptor).  However, these two options are in an if/else block conditioned on the presence of the artifact file (directory), so if that value is not null, the second check never occurs.

Following the getClassesFile() logic (called from line 5008, executing line 1709) we see in line 1737 that if the output directory exists, that directory is returned and the code never reaches the null return at line 1741.

Possible fix:  replace the if/else conditional with an if conditional, and condition the second branch (currently "else") to be a second check on whether `result` is null.

 

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**Affects:** 3.4.1

Start with AbstractJavadocMojo.java around lines 4999-5096 and trace getClassesFile() around line 1709. Reproduce the failure with the linked foo project by invoking maven-clean-plugin during initialize, then run mvn javadoc:javadoc or mvn javadoc:jar. Done means the module-info.java project builds successfully without requiring an Automatic-Module-Name.

Written by the indexing model from the issue text.