Summary
-------

Revise the specification of the `java.lang.System::getProperties` method to allow the values of specific system properties to be undefined, and to allow the system property `java.vendor.version` to be undefined.

Problem
-------

The system property `java.vendor.version` was introduced by 
[JEP 322: Time-Based Release Versioning](https://openjdk.java.net/jeps/322#System-properties) in JDK 10.  The JEP says:

  - `java.vendor.version` — An implementor-specific product
    version string, optionally assigned by the individual or organization
    that produces a specific implementation.  If not assigned at build
    time then it has no value; otherwise, its value is a non-empty string
    that matches the regular expression `\p{Graph}+`.

The specification of the `java.lang.System::getProperties` method contains a table of all of the standard system properties.  The `java.vendor.version` property was added to that table when JEP 322 was implemented.  That specification mandates, however, that "this set of system properties always includes values for the following keys," which is true of every property listed except for `java.vendor.version`.

Solution
--------

Revise the specification of the `java.lang.System::getProperties` method to allow the values of specific system properties to be undefined, and to allow the system property `java.vendor.version` to be undefined.

Specification
-------------

```
diff --git a/src/java.base/share/classes/java/lang/System.java b/src/java.base/share/classes/java/lang/System.java
--- a/src/java.base/share/classes/java/lang/System.java
+++ b/src/java.base/share/classes/java/lang/System.java
@@ -618,8 +618,9 @@
      * {@link #getProperty(String)} method is returned as a
      * {@code Properties} object. If there is no current set of
      * system properties, a set of system properties is first created and
-     * initialized. This set of system properties always includes values
-     * for the following keys:
+     * initialized. This set of system properties includes a value
+     * for each of the following keys unless the description of the associated
+     * value indicates that the value is optional:
      * <table class="striped" style="text-align:left">
      * <caption style="display:none">Shows property keys and associated values</caption>
      * <thead>
@@ -639,7 +640,7 @@
      * <tr><th scope="row">{@systemProperty java.vendor.url}</th>
      *     <td>Java vendor URL</td></tr>
      * <tr><th scope="row">{@systemProperty java.vendor.version}</th>
-     *     <td>Java vendor version</td></tr>
+     *     <td>Java vendor version <em>(optional)</em> </td></tr>
      * <tr><th scope="row">{@systemProperty java.home}</th>
      *     <td>Java installation directory</td></tr>
      * <tr><th scope="row">{@systemProperty java.vm.specification.version}</th>
```

Specdiff: https://cr.openjdk.java.net/~mr/rev/8179727/specdiff/

Javadoc output: https://cr.openjdk.java.net/~mr/rev/8179727/java.base/java/lang/System.html#getProperties%28%29