Enhance DICOM Parser to Handle Missing Attributes

[PranshuLakhotia]

The current DICOM parser in DICOMHawk is not robust enough to handle DICOM files with missing attributes. This can lead to AttributeErrors and potential crashes when processing diverse DICOM files.

Specific problems:

1. The parser assumes all DICOM files will have standard attributes like PatientID, PatientName, and PatientBirthDate.
2. When these attributes are missing, the parser raises an AttributeError, potentially crashing the application.
3. The current implementation doesn't provide a clear way to identify missing data in the parsed output.

Proposed solution:
Modify the parser to use getattr() with default values for all attributes.
Update the parser to return 'N/A' for any missing attributes.
Enhance test cases to verify the parser's behavior with missing attributes.

[PranshuLakhotia]

@gtheodoridis raising a PR for the implementation of the proposed solution.