fix: Handle optional ContentSequence for NUM content items

[Devu-trenser]

Context

This MR fixes an issue in NUM content item handling where ContentSequence was assumed to always be present.

In DICOM SR (e.g. TID 300 / TID 1501), NUM items may represent purely numeric measurements without any spatial reference (SCOORD / SCOORD3D). Attempting to access ValueType unconditionally caused unnecessary warnings and could skip valid numeric measurements.

This change makes spatial processing conditional on the presence of ContentSequence, aligning the implementation with the DICOM SR specification.

Changes & Results

Guard access to ContentSequence before reading ValueType

Testing

This PR is related to dcmjs-org/dcmjs#460 and cornerstonejs/cornerstone3D#2430

Verified that SR files are loaded correctly.

Checklist

PR

• My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• My code has been well-documented (function documentation, inline comments,
  etc.)

Public Documentation Updates

• The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• [] OS:
• [] Node version:
• [] Browser:

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