Don't raise IndexError when GPS IFD tag is missing

When images are missing this tag (this is apparently common for many
images generated by phones), running them through
`ImageOps.exif_transpose` raises `IndexError`.

The root cause here is they're missing some tags in the EXIF metadata,
and this isn't handled very gracefully.

Fixes #4215

Tests/test_file_jpeg.py

@@ -735,6 +735,14 @@ class TestFileJpeg:
         # Assert the entire file has not been read
         assert 0 < buffer.max_pos < size
 
+    def test_missing_gps_ifd(self):
+        # This image is missing its gps ifd data.
+        with Image.open("Tests/images/exif-gps-ifd-missing.jpeg") as im:
+
+            # This should return the default, and not a IndexError when a tag is
+            # missing.
+            assert im.info.get("dpi") == (72, 72)
+
 
 @pytest.mark.skipif(not is_win32(), reason="Windows only")
 @skip_unless_feature("jpg")

src/PIL/TiffImagePlugin.py

@@ -587,15 +587,20 @@ class ImageFileDirectory_v2(MutableMapping):
                 TiffTags.SIGNED_RATIONAL,
             ]:  # rationals
                 values = (values,)
-            try:
+
+            if len(values) == 1:
                 (dest[tag],) = values
-            except ValueError:
+            elif len(values) > 1:
                 # We've got a builtin tag with 1 expected entry
                 warnings.warn(
                     "Metadata Warning, tag %s had too many entries: %s, expected 1"
                     % (tag, len(values))
                 )
                 dest[tag] = values[0]
+            else:
+                warnings.warn(
+                    "Metadata Warning, tag %s had no entries, expected 1" % tag
+                )
 
         else:
             # Spec'd length > 1 or undefined