[GR-47694] Accept empty java.vendor.version property in fallback version printer

The JDK build system does not enforce it to be set. The default is empty
string and, if it's indeed empty string, it won't be set at all.

See
* https://github.com/openjdk/jdk/blob/b093880acd89d8d0bccd4b8b260b721f4dcfc161/make/autoconf/jdk-version.m4#L479-L493
* https://github.com/openjdk/jdk/blob/19e2c8c321823c056091e6e9f6c3d0db7ba9ec2b/src/java.base/share/classes/java/lang/VersionProps.java.template#L106-L107

Fixes the following issue:
```
$ mx --env jvm espresso-standalone -version
espresso 20 2023-03-21
org.graalvm.launcher.Launcher$AbortException
Caused by: java.lang.NullPointerException: Cannot invoke "String.isEmpty()" because "vendorVersion" is null
        at com.oracle.truffle.espresso.launcher.EspressoLauncher.printVersionFallback(EspressoLauncher.java:646)
        at com.oracle.truffle.espresso.launcher.EspressoLauncher.launch(EspressoLauncher.java:571)
        at org.graalvm.launcher.AbstractLanguageLauncher.launch(AbstractLanguageLauncher.java:296)
        at org.graalvm.launcher.AbstractLanguageLauncher.launch(AbstractLanguageLauncher.java:121)
        at com.oracle.truffle.espresso.launcher.EspressoLauncher.main(EspressoLauncher.java:49)
```

Author: lewurm

espresso/src/com.oracle.truffle.espresso.launcher/src/com/oracle/truffle/espresso/launcher/EspressoLauncher.java

@@ -622,7 +622,7 @@ private void printVersionFallback(Context context) {
         String javaVersion = system.invokeMember("getProperty", "java.version").asString();
         String javaVersionDate = system.invokeMember("getProperty", "java.version.date").asString();
         String debugLevel = system.invokeMember("getProperty", "jdk.debug", "release").asString();
-        String vendorVersion = system.invokeMember("getProperty", "java.vendor.version").asString();
+        String vendorVersion = system.invokeMember("getProperty", "java.vendor.version", "").asString();
         String javaRuntimeName = system.invokeMember("getProperty", "java.runtime.name").asString();
         String javaRuntimeVersion = system.invokeMember("getProperty", "java.runtime.version").asString();
         boolean isLTS = javaRuntimeVersion.contains("LTS");