Use "string" instead of pyarrow.string() in case pyarrow not installed

Fixes `AttributeError: 'NoneType' object has no attribute 'string'`

Author: weiji14

pygmt/tests/test_clib_put_strings.py

@@ -23,7 +23,7 @@
         pytest.param(np.array, {"dtype": str}, id="str"),
         pytest.param(
             getattr(pa, "array", None),
-            {"type": pa.string()},
+            {"type": "string"},  # pa.string()
             marks=skip_if_no(package="pyarrow"),
             id="pyarrow",
         ),

pygmt/tests/test_clib_virtualfiles.py

@@ -238,7 +238,7 @@ def test_virtualfile_from_vectors(dtypes):
         pytest.param(np.array, {"dtype": object}, id="object"),
         pytest.param(
             getattr(pa, "array", None),
-            {"type": pa.string()},
+            {"type": "string"},  # pa.string()
             marks=skip_if_no(package="pyarrow"),
             id="pyarrow",
         ),