Fix parse errors for empty build files

Document::rangeBetween would return `null` if the document was empty,
but I wasn't checking for that in ToSmithyNode (which creates parse
events).

The reason the range is `null` is because Document.lineOfIndex returns
oob for an index of 0 into an empty document. Makes sense, as an empty
document has no lines. I updated a DocumentTest to clarify this
behavior.

Author: milesziemer

src/main/java/software/amazon/smithy/lsp/project/ToSmithyNode.java

@@ -6,6 +6,7 @@
 package software.amazon.smithy.lsp.project;
 
 import java.util.List;
+import org.eclipse.lsp4j.Range;
 import software.amazon.smithy.lsp.document.Document;
 import software.amazon.smithy.lsp.protocol.LspAdapter;
 import software.amazon.smithy.lsp.syntax.Syntax;
@@ -104,6 +105,10 @@ yield switch (value) {
     }
 
     private SourceLocation nodeStartSourceLocation(Syntax.Node node) {
-        return LspAdapter.toSourceLocation(path, document.rangeBetween(node.start(), node.end()));
+        Range range = document.rangeBetween(node.start(), node.end());
+        if (range == null) {
+            range = LspAdapter.origin();
+        }
+        return LspAdapter.toSourceLocation(path, range);
     }
 }

src/test/java/software/amazon/smithy/lsp/document/DocumentTest.java

@@ -293,6 +293,7 @@ public void getsLineOfIndex() {
         Document leadingAndTrailingWs = makeDocument("\nabc\n");
         Document threeLine = makeDocument("abc\ndef\nhij\n");
 
+        assertThat(empty.lineOfIndex(0), is(-1)); // empty has no lines, so oob
         assertThat(empty.lineOfIndex(1), is(-1)); // oob
         assertThat(single.lineOfIndex(0), is(0)); // start
         assertThat(single.lineOfIndex(2), is(0)); // end

src/test/java/software/amazon/smithy/lsp/project/ProjectConfigTest.java

@@ -24,6 +24,7 @@
 import java.util.List;
 import java.util.function.Supplier;
 import java.util.stream.Collectors;
+import org.eclipse.lsp4j.Position;
 import org.junit.jupiter.api.Test;
 import software.amazon.smithy.build.model.MavenRepository;
 import software.amazon.smithy.cli.dependencies.DependencyResolver;
@@ -32,6 +33,7 @@
 import software.amazon.smithy.lsp.ServerState;
 import software.amazon.smithy.lsp.TextWithPositions;
 import software.amazon.smithy.lsp.document.Document;
+import software.amazon.smithy.lsp.protocol.LspAdapter;
 
 public class ProjectConfigTest {
     @Test
@@ -78,6 +80,21 @@ public void mergesBuildExts() {
         assertThat(config.maven().getDependencies(), containsInAnyOrder("foo"));
     }
 
+    @Test
+    public void handlesEmptyFiles() {
+        var root = Path.of("foo");
+        var buildFiles = createBuildFiles(root, BuildFileType.SMITHY_BUILD, "");
+        var result = load(root, buildFiles);
+
+        var smithyBuild = buildFiles.getByType(BuildFileType.SMITHY_BUILD);
+        assertThat(smithyBuild, notNullValue());
+        assertThat(result.events(), containsInAnyOrder(allOf(
+                eventWithId(equalTo("Model")),
+                eventWithMessage(containsString("Error parsing JSON")),
+                eventWithSourceLocation(equalTo(LspAdapter.toSourceLocation(smithyBuild.path(), new Position(0, 0))))
+        )));
+    }
+
     @Test
     public void validatesSmithyBuildJson() {
         var text = TextWithPositions.from("""