Clearing system property user.dir causes File methods to throw NPE

If a File is relative then operations that require current directory, e.g. getAbsoluteFile(), getCanonicalPath(), fail with NPE if user.dir property has been previously cleared (via clearProperty(String) or setProperties(Properties)).
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Example:
jshell> new File("foo/bar").getAbsoluteFile()
$1 ==> d:\\foo\bar

jshell> System.clearProperty("user.dir");
$2 ==> "d:\\"

jshell> new File("foo/bar").getAbsoluteFile()
| java.lang.NullPointerException thrown:
| at WinNTFileSystem.normalize (WinNTFileSystem.java:83)
| at WinNTFileSystem.getUserPath (WinNTFileSystem.java:349)
| at WinNTFileSystem.resolve (WinNTFileSystem.java:312)
| at File.getAbsolutePath (File.java:556)
| at File.getAbsoluteFile (File.java:572)
| at (#3:1)
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Note that with empty user.dir calls don't fail.
Example:
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jshell> System.setProperty("user.dir", "");
$4 ==> null

jshell> new File("foo/bar").getAbsoluteFile()
$5 ==> \foo\bar
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