tests: cover leading empty fields in base_str_split

lib/bash/str/README.md

@@ -71,6 +71,8 @@ base_str_join joined "|" parts
 - Predicate helpers require exactly two arguments, return shell status, and do
   not print output.
 - `base_str_split` preserves empty fields between repeated delimiters.
+- `base_str_split` preserves an empty first field when the input begins with the
+  separator.
 - `base_str_split` preserves a trailing empty field when the input ends with the
   separator.
 - `base_str_join` preserves empty array elements, including trailing empty elements.

lib/bash/str/tests/lib_str.bats

@@ -248,6 +248,17 @@ EOF
     [ "${parts[2]}" = "" ]
 }
 
+@test "base_str_split preserves a leading empty field before the first separator" {
+    local -a parts=()
+
+    base_str_split parts ",alpha,beta" ","
+
+    [ "${#parts[@]}" -eq 3 ]
+    [ "${parts[0]}" = "" ]
+    [ "${parts[1]}" = "alpha" ]
+    [ "${parts[2]}" = "beta" ]
+}
+
 @test "base_str_split can store results in an array named fields" {
     local -a fields=()