PMD is currently not working properly for Java classes that use [unnamed
packages](https://docs.oracle.com/javase/specs/jls/se11/html/jls-7.html#jls-7.4.2).
Consider the following Java class that does not contain a `package`
declaration:
```java
public class App {
String getGreeting() {
return "Hello world.";
}
static void main(String... args) {
System.out.println(new App().getGreeting());
}
}
```
Running PMD in the command line agaist the Java class above produces an
output with empty string `""` in the `"Package"` column:
```sh
$ pmd -R category/java/bestpractices.xml -f csv -d './src/main/java/App.java'
Oct 02, 2018 9:10:39 PM net.sourceforge.pmd.PMD processFiles
WARNING: This analysis could be faster, please consider using Incremental Analysis: https://pmd.github.io/pmd-6.7.0/pmd_userdocs_incremental_analysis.html
"Problem","Package","File","Priority","Line","Description","Rule set","Rule"
"1","","/Users/diego/Projects/github.com/dlresende/kata-fizz-buzz/src/main/java/App.java","2","7","System.out.println is used","Best Practices","SystemPrintln"
```
But the pmd.vim handler's current pattern refuses everything coming
from a Java class that does not have a package name (2nd column):
```vim
let l:pattern = '"\(\d\+\)",".\+","\(.\+\)","\(\d\+\)","\(\d\+\)","\(.\+\)","\(.\+\)","\(.\+\)"$'
```
The solution I am proposing is to also accept empty strings as package names.
