[Bug]: NPE on getEndColumn

### Describe the bug

Even though the source position of a CtCompilationUnit is valid, calling getEndColumn() throws an exception.

I'm running this in a JUnit test with a VirtualFile. If the file contains newlines I get the error, otherwise I don't.

Note that I extracted this from another unit test, so the bottom of the stacktrace does not match what you would get.

FWIW, the end position is completely bogus. I get 2147483646 for an end position.

### Source code you are trying to analyze/transform

```Java
class Source {
// newline is essential; seems to work fine without newline
}
```

### Source code for your Spoon processing

```Java
void test(String sourceCode) {
Launcher launcher = new Launcher();
// Load from test resources
launcher.addInputResource(new VirtualFile(sourceCode, "x/y/z/Source.java"));
launcher.getEnvironment().setNoClasspath(true);
launcher.getEnvironment().setComplianceLevel(17);
launcher.buildModel();

assert(1== launcher.getFactory().CompilationUnit().getMap().size());
CtCompilationUnit ct = launcher.getFactory().CompilationUnit().getMap().values().iterator().next();
assert(ct.getPosition().isValidPosition());
ct.getPosition().getEndColumn();
}
```

### Actual output

```Java
java.lang.NullPointerException: Cannot invoke "String.charAt(int)" because "source" is null
at spoon.support.reflect.cu.position.SourcePositionImpl.searchColumnNumber(SourcePositionImpl.java:87)
at spoon.support.reflect.cu.position.SourcePositionImpl.getEndColumn(SourcePositionImpl.java:132)
at us.str.axle.spoon.callbacks.Standard_JUnit.execCallback(Standard_JUnit.java:105)
at us.str.axle.spoon.callbacks.Standard_JUnit.testSimple(Standard_JUnit.java:130)
```

### Expected output

```Java

```

### Spoon Version

11.3.0

### JVM Version

Java17

### What operating system are you using?

Alma Linux 9

Start with spoon.support.reflect.cu.position.SourcePositionImpl.java, especially searchColumnNumber and getEndColumn, and reproduce the issue with the provided Launcher, VirtualFile, and newline-containing Java source. Done means the valid CtCompilationUnit position no longer throws a NullPointerException when getEndColumn() is called and the reported end position is no longer the bogus value.

Written by the indexing model from the issue text.