handle empty environment variable case

Author: musm

base/client.jl

@@ -31,7 +31,11 @@ stackframe_lineinfo_color() = repl_color("JULIA_STACKFRAME_LINEINFO_COLOR", :bol
 stackframe_function_color() = repl_color("JULIA_STACKFRAME_FUNCTION_COLOR", :bold)
 
 function repl_cmd(cmd, out)
-    shell = shell_split(get(ENV, "JULIA_SHELL", Sys.iswindows() ? "cmd" : get(ENV, "SHELL", "/bin/sh")))
+    shell = get(ENV, "JULIA_SHELL", nothing)
+    if shell === nothing || isempty(shell)
+        shell = Sys.iswindows() ? "cmd" : get(ENV, "SHELL", "/bin/sh")
+    end
+    shell = shell_split(shell)
     shell_name = Base.basename(shell[1])
     if Sys.iswindows()
         shell_name = lowercase(splitext(shell_name)[1]) # canonicalize for comparisons below