fix: handle empty strings in price data parsing (issue #49)

Apply consistent empty-string handling across main.py and scanner.py
to prevent ValueError when KIS API returns empty strings.

Changes:
- src/main.py:110 - Add 'or "0"' for current_price parsing
- src/analysis/scanner.py:86-87 - Add 'or "0"' for price/volume parsing
- tests/test_main.py - Add test_overseas_price_empty_string
- tests/test_volatility.py - Add test_scan_stock_overseas_empty_price

Before: ValueError crashes trading cycle
After: Empty strings default to 0.0, trading continues

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/analysis/scanner.py

@@ -83,8 +83,8 @@ class MarketScanner:
                 # Convert to orderbook-like structure
                 orderbook = {
                     "output1": {
-                        "stck_prpr": price_data.get("output", {}).get("last", "0"),
-                        "acml_vol": price_data.get("output", {}).get("tvol", "0"),
+                        "stck_prpr": price_data.get("output", {}).get("last", "0") or "0",
+                        "acml_vol": price_data.get("output", {}).get("tvol", "0") or "0",
                     }
                 }