Use correct syntax for empty YAML sequence

Unlike the other inputs (`platforms`, `sketch-paths`) that take a "sequence" (AKA array, list) type YAML document as
input, because a sketch may have no library dependencies it is valid and common to pass an empty sequence via the
`libraries` input.

Due to the need to maintain backwards compatibility with the original short-sighted design of the action API, which
didn't consider the action being used with any project type other than libraries, it is necessary for the user to do
this in their workflow rather than simply omitting the `libraries` input.

Previously the documentation and tests used "-" for this purpose. That is supported, but is not an empty sequence, but
rather a sequence consisting of a single null element. The only way to specify an empty sequence in YAML is `[]`.

Author: per1234

.github/workflows/test-integration.yml

@@ -72,7 +72,7 @@ jobs:
             libraries: |
               # The official Servo library is not compatible with ESP8266, but that platform has a bundled Servo lib
               # so there are no library dependencies
-              -
+              []
 
     steps:
       - name: Checkout
@@ -348,7 +348,7 @@ jobs:
           fqbn: arduino:avr:uno
           github-token: ${{ secrets.GITHUB_TOKEN }}
           libraries: |
-            -
+            []
           sketch-paths: |
             - ${{ env.TESTDATA_SKETCHES_PATH }}/Error
 

README.md

@@ -129,7 +129,7 @@ Keys:
 [YAML](https://en.wikipedia.org/wiki/YAML)-format list of library dependencies to install.
 
 **Default**: `"- source-path: ./"`
-This causes the repository to be installed as a library. If there are no library dependencies and you want to override the default, set the `libraries` input to an empty list (`- libraries: '-'`).
+This causes the repository to be installed as a library. If there are no library dependencies and you want to override the default, set the `libraries` input to an empty list (`- libraries: '[]'`).
 
 Libraries are installed under the Arduino user folder at `~/Arduino/libraries`.
 

compilesketches/tests/test_compilesketches.py

@@ -1070,6 +1070,7 @@ def __init__(self):
             [],
         ),
         ("-", [], [], [], []),
+        ("[]", [], [], [], []),
         (
             "- " + compilesketches.CompileSketches.dependency_name_key + ": foo",
             [{compilesketches.CompileSketches.dependency_name_key: "foo"}],