Allow empty spack environments when constructing hash

When building an experiment / workspace inventory, previously it was
required that if an environment had a spack.yaml, it also created a
spack.lock (or Ramble would error).

This commit allows spack.yaml files that have no packages (And thus,
won't concretized into a spack.lock) without throwing an error.

lib/ramble/ramble/spack_runner.py

@@ -602,7 +602,7 @@ def inventory_hash(self, require_exist=False):
         contents_to_hash = None
 
         if not self.dry_run:
-            if not lock_exists and require_exist:
+            if not lock_exists and (require_exist or len(self.env_contents) == 0):
                 logger.die(
                     'spack.lock file does not exist in environment '
                     f'{self.env_path}\n'

lib/ramble/ramble/test/workspace_hashing/unsetup_workspace_cannot_analyze.py

@@ -49,8 +49,13 @@ def test_unsetup_workspace_cannot_analyze(mutable_config,
                   MY_VAR: 'TEST'
   spack:
     concretized: true
-    packages: {}
-    environments: {}
+    packages:
+      zlib:
+        spack_spec: zlib
+    environments:
+      zlib:
+        packages:
+        - zlib
 """
     workspace_name = 'test_unsetup_workspace_cannot_analyze'
     with ramble.workspace.create(workspace_name) as ws: