fix: preserve empty string values in query parameter tuples

Resolves issue where query parameters with explicit empty string values were ambiguously handled during URL serialization.

Author: pactender

test_query_params.py

@@ -0,0 +1,23 @@
+import httpx
+import pytest
+
+
+def test_query_params_preserve_empty_string():
+    params = [("filter", "active"), ("tag", "")]
+    url = httpx.URL("https://example.com/api", params=params)
+    assert str(url) == "https://example.com/api?filter=active&tag="
+
+
+def test_query_params_tuple_with_multiple_empty_values():
+    params = [("key1", ""), ("key2", "value"), ("key3", "")]
+    request = httpx.Request("GET", "https://example.com/items", params=params)
+    assert str(request.url) == "https://example.com/items?key1=&key2=value&key3="
+
+
+def test_query_params_none_vs_empty_string():
+    # None should omit value, empty string should preserve key=
+    params_with_empty = [("key", "")]
+    params_with_none = [("key", None)]
+
+    assert str(httpx.URL("https://example.com", params=params_with_empty)) == "https://example.com?key="
+    assert str(httpx.URL("https://example.com", params=params_with_none)) == "https://example.com?key"