Fix failing unit-test test_wsireader (#7905)

Conversion of units when the unit if '' caused the test to fail.

```bash
pytest -k tiff
```

FAILED
tests/test_wsireader.py::TestTiffFile::test_resolution_mpp_0__home_johnsonhj_src_MONAI_tests_testing_data_temp_wsi_generic_tiff_tiff
- ValueError: Currently, it only supports length conversion but `` is
given.

FAILED
tests/test_wsireader.py::TestTiffFile::test_resolution_mpp_0__home_johnsonhj_src_MONAI_tests_testing_data_temp_wsi_generic_tiff_tiff
- AttributeError: 'ConvertUnits' object has no attribute
'conversion_factor'

### Description

Fixes a failing test.  

### Types of changes
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [x] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [x] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.

Signed-off-by: Hans Johnson <hans-johnson@uiowa.edu>
Co-authored-by: YunLiu <55491388+KumoLiu@users.noreply.github.com>

Author: hjmjohnson

monai/data/wsi_reader.py

@@ -1098,7 +1098,7 @@ def get_mpp(self, wsi, level: int) -> tuple[float, float]:
             unit = wsi.pages[level].tags.get("ResolutionUnit")
             if unit is not None:
                 unit = str(unit.value)[8:]
-            else:
+            if unit is None or len(unit) == 0:
                 warnings.warn("The resolution unit is missing. `micrometer` will be used as default.")
                 unit = "micrometer"
 

monai/utils/misc.py

@@ -814,7 +814,7 @@ def __init__(self, input_unit: str, target_unit: str) -> None:
                 "Both input and target units should be from the same quantity. "
                 f"Input quantity is {input_base} while target quantity is {target_base}"
             )
-        self._calculate_conversion_factor()
+        self.conversion_factor = self._calculate_conversion_factor()
 
     def _get_valid_unit_and_base(self, unit):
         unit = str(unit).lower()
@@ -841,7 +841,7 @@ def _calculate_conversion_factor(self):
             return 1.0
         input_power = self._get_unit_power(self.input_unit)
         target_power = self._get_unit_power(self.target_unit)
-        self.conversion_factor = 10 ** (input_power - target_power)
+        return 10 ** (input_power - target_power)
 
     def __call__(self, value: int | float) -> Any:
         return float(value) * self.conversion_factor