GMT_IMAGE: Remove 'nan_value' from the doctest since it depends on GM…

…T versions

pygmt/datatypes/image.py

@@ -18,7 +18,7 @@ class _GMT_IMAGE(ctp.Structure):  # noqa: N801
     >>> from pygmt.clib import Session
     >>> with Session() as lib:
     ...     with lib.virtualfile_out(kind="image") as voutimg:
-    ...         lib.call_module("read", ["@earth_day_01d", voutimg, "-Ti"])
+    ...         lib.call_module("read", ["@earth_day_01d_p", voutimg, "-Ti"])
     ...         # Read the image from the virtual file
     ...         image = lib.read_virtualfile(vfname=voutimg, kind="image").contents
     ...         # The image header
@@ -34,7 +34,7 @@ class _GMT_IMAGE(ctp.Structure):  # noqa: N801
     ...         print(header.nm, header.size, header.complex_mode)
     ...         print(header.type, header.n_bands, header.mx, header.my)
     ...         print(header.pad[:])
-    ...         print(header.mem_layout, header.nan_value, header.xy_off)
+    ...         print(header.mem_layout, header.xy_off)
     ...         # Image-specific attributes.
     ...         print(image.type, image.n_indexed_colors)
     ...         # The x and y coordinates
@@ -58,7 +58,7 @@ class _GMT_IMAGE(ctp.Structure):  # noqa: N801
     64800 66976 0
     0 3 364 184
     [2, 2, 2, 2]
-    b'BRPa' 0.0 0.5
+    b'BRPa' 0.5
     1 0
     >>> x
     [-179.5, -178.5, ..., 178.5, 179.5]