🎨 Warn about empty password/token action input

Before this patch, the warning would say that the token was
expected to start with `pypi-` but it may be unobvious. With this
change, the end-users are warned when they're passing a completely
empty password value.

Fixes #25.

twine-upload.sh

@@ -23,6 +23,15 @@ if [[
     ! "$INPUT_PASSWORD" =~ ^pypi-
   ]]
 then
+    if [[ -z "$INPUT_PASSWORD" ]]; then
+        echo \
+            ::warning file='# >>' PyPA publish to PyPI GHA'%3A' \
+            EMPTY TOKEN \
+            '<< ':: \
+            It looks like you have not passed a password or it \
+            is otherwise empty. Please verify that you have passed it \
+            directly or, preferably, through a secret.
+    else
         echo \
             ::warning file='# >>' PyPA publish to PyPI GHA'%3A' \
             POTENTIALLY INVALID TOKEN \
@@ -32,6 +41,7 @@ then
             not start with '"pypi-"' as it typically should. This may \
             cause an authentication error. Please verify that you have \
             copied your token properly if such an error occurs.
+    fi
 fi
 
 if ( ! ls -A ${INPUT_PACKAGES_DIR%%/}/*.tar.gz &> /dev/null && \