Do not add external reference if URL is explicitly empty

[mtgag]

Projects that inherit properties like scm or url from parent pom files always include the website or vcs external references like that

  "externalReferences" : [
    {
      "type" : "website",
      "url" : "URL value"
    },
    {
      "type" : "vcs",
      "url" : "URL value"
    }
  ],

One option is to provide empty values in child pom.xml like that:

<scm>
    <url></url>
</scm>
<url></url>

which leads to this bom file:

  "externalReferences" : [
    {
      "type" : "website",
      "url" : ""
    },
    {
      "type" : "vcs",
      "url" : ""
    }
  ],

and another option is to place correct URLs, which in some scenarios may not be desired.

Therefore the bom file may appear like that:

1. The external parameters are present and have the parent values, or
2. The external parameters are present and have empty values, or
3. The external parameters are present and have correct (or placeholder) values

Do other options exist?

Would it be meaningful to change this code:

cyclonedx-maven-plugin/src/main/java/org/cyclonedx/maven/DefaultModelConverter.java

Line 272 in 47742b1

if (url == null || doesComponentHaveExternalReference(component, referenceType)) {

to:

    if (url == null || url.isEmpty() || url.trim().length() == 0 || doesComponentHaveExternalReference(component, referenceType)) {
        return;
    }

to allow not to add an externalReference for a certain type if the URL provided for this type is explicitly empty?

[hboutemy]

you're right that the intent of url == null is more isEmpty(url)
can you provide a PR, please?

[hboutemy]

fixed by #396