Clarify behaviour of readAll at the end of a file

This should return an empty string, to match PUC Lua.

projects/core/src/main/java/dan200/computercraft/core/apis/handles/AbstractHandle.java

@@ -177,9 +177,9 @@ public abstract class AbstractHandle {
     /**
      * Read the remainder of the file.
      *
-     * @return The file, or {@code null} if at the end of it.
+     * @return The remaining contents of the file, or {@code null} in the event of an error.
      * @throws LuaException If the file has been closed.
-     * @cc.treturn string|nil The remaining contents of the file, or {@code nil} if we are at the end.
+     * @cc.treturn string|nil The remaining contents of the file, or {@code nil} in the event of an error.
      * @cc.since 1.80pr1
      */
     @Nullable

projects/core/src/test/resources/test-rom/spec/apis/fs_spec.lua

@@ -200,6 +200,14 @@ describe("The fs library", function()
                 handle.close()
             end)
 
+            it("reading an empty file returns nil", function()
+                local file = create_test_file ""
+
+                local handle = fs.open(file, mode)
+                expect(handle.read()):eq(nil)
+                handle.close()
+            end)
+
             it("can read a line of text", function()
                 local file = create_test_file "some\nfile\r\ncontents\n\n"
 
@@ -223,6 +231,16 @@ describe("The fs library", function()
                 expect(handle.readLine(true)):eq(nil)
                 handle.close()
             end)
+
+            it("readAll always returns a string", function()
+                local contents = "some\nfile\ncontents"
+                local file = create_test_file "some\nfile\ncontents"
+
+                local handle = fs.open(file, mode)
+                expect(handle.readAll()):eq(contents)
+                expect(handle.readAll()):eq("")
+                handle.close()
+            end)
         end
 
         describe("reading", function()