Fix panic when using join_selections_space (#9783)

Joining lines with Alt-J does not properly select the inserted spaces
when the selection contains blank lines. In the worst case it panics
with an out of bounds index.

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value:
Char index out of bounds: char index 11, Rope/RopeSlice char length 10'

Steps to reproduce:
* Create a new document
    ```
    a

    b

    c

    d

    e
    ```
* % (Select all)
* Alt-J (join and select the spaces)

2 files changed, 100 insertions, 6 deletions

diff --git a/helix-term/src/commands.rs b/helix-term/src/commands.rs
index bd0a60b7..0b2ea0b8 100644
--- a/helix-term/src/commands.rs
+++ b/helix-term/src/commands.rs
@@ -4372,16 +4372,27 @@ fn join_selections_impl(cx: &mut Context, select_space: bool) {
 
     // select inserted spaces
     let transaction = if select_space {
+        let mut offset: usize = 0;
         let ranges: SmallVec<_> = changes
             .iter()
-            .scan(0, |offset, change| {
-                let range = Range::point(change.0 - *offset);
-                *offset += change.1 - change.0 - 1; // -1 because cursor is 0-sized
-                Some(range)
+            .filter_map(|change| {
+                if change.2.is_some() {
+                    let range = Range::point(change.0 - offset);
+                    offset += change.1 - change.0 - 1; // -1 adjusts for the replacement of the range by a space
+                    Some(range)
+                } else {
+                    offset += change.1 - change.0;
+                    None
+                }
             })
             .collect();
-        let selection = Selection::new(ranges, 0);
-        Transaction::change(text, changes.into_iter()).with_selection(selection)
+        let t = Transaction::change(text, changes.into_iter());
+        if ranges.is_empty() {
+            t
+        } else {
+            let selection = Selection::new(ranges, 0);
+            t.with_selection(selection)
+        }
     } else {
         Transaction::change(text, changes.into_iter())
     };
diff --git a/helix-term/tests/test/commands.rs b/helix-term/tests/test/commands.rs
index e52b142c..1172a798 100644
--- a/helix-term/tests/test/commands.rs
+++ b/helix-term/tests/test/commands.rs
@@ -526,3 +526,86 @@ async fn test_join_selections() -> anyhow::Result<()> {
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread")]
+async fn test_join_selections_space() -> anyhow::Result<()> {
+    // join with empty lines panic
+    test((
+        platform_line(indoc! {"\
+            #[a
+
+            b
+
+            c
+
+            d
+
+            e|]#
+        "}),
+        "<A-J>",
+        platform_line(indoc! {"\
+            a#[ |]#b#( |)#c#( |)#d#( |)#e
+        "}),
+    ))
+    .await?;
+
+    // normal join
+    test((
+        platform_line(indoc! {"\
+            #[a|]#bc
+            def
+        "}),
+        "<A-J>",
+        platform_line(indoc! {"\
+            abc#[ |]#def
+        "}),
+    ))
+    .await?;
+
+    // join with empty line
+    test((
+        platform_line(indoc! {"\
+            #[a|]#bc
+
+            def
+        "}),
+        "<A-J>",
+        platform_line(indoc! {"\
+            #[a|]#bc
+            def
+        "}),
+    ))
+    .await?;
+
+    // join with additional space in non-empty line
+    test((
+        platform_line(indoc! {"\
+            #[a|]#bc
+
+                def
+        "}),
+        "<A-J><A-J>",
+        platform_line(indoc! {"\
+            abc#[ |]#def
+        "}),
+    ))
+    .await?;
+
+    // join with retained trailing spaces
+    test((
+        platform_line(indoc! {"\
+            #[aaa   
+
+            bb  
+
+            c |]#
+        "}),
+        "<A-J>",
+        platform_line(indoc! {"\
+            aaa   #[ |]#bb  #( |)#c 
+        "}),
+    ))
+    .await?;
+
+    Ok(())
+}