cmListCommand: handle empty lists for list(REMOVE_AT)

Treat an empty list as a list with no valid bounds and return an error
message indicating that any given indices are out-of-bounds.

Tests/RunCMake/list/EmptyRemoveAt0-stderr.txt

@@ -1,4 +1,4 @@
 CMake Error at EmptyRemoveAt0.cmake:2 \(list\):
-  list REMOVE_AT given empty list
+  list index: mylist, 0 out of range \(0, 0\)
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)$

Tests/RunCMake/list/REMOVE_AT-EmptyList-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/list/REMOVE_AT-EmptyList-stderr.txt

@@ -0,0 +1,4 @@
+^CMake Error at REMOVE_AT-EmptyList.cmake:2 \(list\):
+  list index: nosuchlist, 0 out of range \(0, 0\)
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)$

Tests/RunCMake/list/REMOVE_AT-EmptyList.cmake

@@ -0,0 +1,6 @@
+set(nosuchlist "")
+list(REMOVE_AT nosuchlist 0)
+if (NOT DEFINED nosuchlist OR NOT nosuchlist STREQUAL "")
+  message(FATAL_ERROR
+    "list(REMOVE_AT) modified our list")
+endif ()

Tests/RunCMake/list/REMOVE_AT-NotList-stderr.txt

@@ -1,4 +1,4 @@
 ^CMake Error at REMOVE_AT-NotList.cmake:2 \(list\):
-  list sub-command REMOVE_AT requires list to be present.
+  list index: nosuchlist, 0 out of range \(0, 0\)
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)$

Tests/RunCMake/list/REMOVE_AT-NotList.cmake

@@ -1,2 +1,6 @@
 unset(nosuchlist)
 list(REMOVE_AT nosuchlist 0)
+if (DEFINED nosuchlist)
+  message(FATAL_ERROR
+      "list(REMOVE_AT) created our list")
+endif ()

Tests/RunCMake/list/RunCMakeTest.cmake

@@ -22,6 +22,8 @@ run_cmake(REMOVE_DUPLICATES-TooManyArguments)
 run_cmake(REVERSE-TooManyArguments)
 run_cmake(SUBLIST-TooManyArguments)
 
+run_cmake(REMOVE_AT-EmptyList)
+
 run_cmake(FILTER-NotList)
 run_cmake(REMOVE_AT-NotList)
 run_cmake(REMOVE_DUPLICATES-NotList)