Support empty multiline strings (#483)

* Support empty multiline strings

Per <https://github.com/kdl-org/kdl/discussions/481>, the grammar doesn't actually allow the first and last newline in a multiline string to be the same (indicating an empty string). This looks like the minimal fix for it.

* Add tests for empty multilines

* remove accidental newline

SPEC.md

@@ -889,7 +889,7 @@ disallowed-keyword-identifiers :=
 quoted-string :=
     '"' single-line-string-body '"' |
     '"""' newline
-    multi-line-string-body newline
+    (multi-line-string-body newline)?
     (unicode-space | ws-escape)* '"""'
 single-line-string-body := (string-character - newline)*
 multi-line-string-body := (('"' | '""')? string-character)*
@@ -909,7 +909,7 @@ raw-string := '#' raw-string-quotes '#' | '#' raw-string '#'
 raw-string-quotes :=
     '"' single-line-raw-string-body '"' |
     '"""' newline
-    multi-line-raw-string-body newline
+    (multi-line-raw-string-body newline)?
     unicode-space* '"""'
 single-line-raw-string-body :=
     '' |

tests/test_cases/expected_kdl/multiline_raw_string_empty.kdl

@@ -0,0 +1 @@
+node ""

tests/test_cases/expected_kdl/multiline_raw_string_empty_indented.kdl

@@ -0,0 +1 @@
+node ""

tests/test_cases/expected_kdl/multiline_string_empty.kdl

@@ -0,0 +1 @@
+node ""

tests/test_cases/expected_kdl/multiline_string_empty_indented.kdl

@@ -0,0 +1 @@
+node ""

tests/test_cases/input/multiline_raw_string_empty.kdl

@@ -0,0 +1,2 @@
+node #"""
+"""#

tests/test_cases/input/multiline_raw_string_empty_indented.kdl

@@ -0,0 +1,2 @@
+node #"""
+	"""#

tests/test_cases/input/multiline_string_empty.kdl

@@ -0,0 +1,2 @@
+node """
+"""

tests/test_cases/input/multiline_string_empty_indented.kdl

@@ -0,0 +1,2 @@
+node """
+	"""