fix(media_player): keep blank app names out of the source list (t49 F1)

t48 F1: source_list stringified the app map's values without filtering, so a
hand-edited map with a blank name ("com.x=") put an empty string in the dropdown.
That entry is worse than useless — it is a dropdown item whose only possible outcome
is an error, because select_source has to reject an empty name as an unknown source.

source_list now drops blank and whitespace-only names. The values themselves are
untouched: no name is rewritten or trimmed here, because the reverse lookup matches on
them, so for a well-formed map the list stays byte-equal to the Browse tree's child
titles (same _app_directory accessor, same order) — pinned by comparing the two
directly, for the user's map and for the curated default. The reverse lookup is
unchanged.

Tests (tests/test_media_sources.py, 3 new; tests/test_standalone_setup.py, 1 new):
a hand-edited map with "" and "   " entries offers only the real names; the offered
list equals the Browse tree titles for well-formed maps (both map and curated
default); the surviving entries still resolve AND launch through select_source, and a
blank name still resolves to nothing; and the editor keeps rejecting "com.x="
(_app_map_from_text returns None, the page answers invalid_app_map and stores nothing
half-way), so a blank name cannot get in that way either.

RED PROOF: with the filter removed, exactly the two blank-name tests fail — the blank
appears in the list, and the entry has no package to resolve to (1 passed / 2 failed;
the byte-equality test passes either way, which is the "well-formed list unchanged"
half of the claim) — while the rest of the file is green; restored, 20 pass.

Verified: 458 passed / 2 skipped, 459 / 1 with androidtvremote2, ruff clean, drift
engine exit 0 under real HA (23 modules, 58 imports, 164 relative imports, 36 call
arguments, 7 awaited HA calls).

Residual, deliberately NOT touched here (out of this repair's scope, and pre-existing):
a blank map entry still reaches the Browse tree as an empty child title, and a PADDED
name ("  TiviMate ", hand-edited only — the editor strips) still fails the reverse
lookup because the map value is matched unstripped. Both are hand-edited-map edges;
say the word and either can be a follow-up.

tests/test_standalone_setup.py

@@ -545,6 +545,31 @@ class TestAppIconColumn:
         assert result["type"] == "form"
         assert result["errors"] == {"base": "invalid_app_map"}
 
+    async def test_a_nameless_row_is_rejected_by_the_editor(self, tmp_path):
+        """t49: the editor already refuses 'com.x=' — a blank name cannot get in.
+
+        The source list filters blanks anyway, because the map can also be
+        hand-edited; this pins the other half: the page will not STORE one.
+        """
+        module = _form_module()
+
+        assert module._app_map_from_text("com.blank=", {}) is None
+        assert module._app_map_from_text("com.blank=   ", {}) is None
+
+        flow = module.NvidiaShieldOptionsFlow()
+        flow.config_entry = SimpleNamespace(options={}, title="Shield")
+        flow.hass = FakeHass(str(tmp_path))
+        flow.async_show_form = lambda **kwargs: {"type": "form", **kwargs}
+        flow.async_create_entry = lambda **kwargs: {"type": "create_entry", **kwargs}
+
+        result = await flow.async_step_init(
+            {"app_name_map": "com.blank=\ncom.netflix.ninja=Netflix"}
+        )
+
+        assert result["type"] == "form"
+        assert result["errors"] == {"base": "invalid_app_map"}
+        assert "app_name_map" not in flow._options  # nothing half-stored
+
 
 class TestChannelEntryOptions:
     """P5-2: digit-delay + optional send-enter on the options page."""