Fix panic formatting processing instructions without attributes

[SAY-5]

Problem

xq core-dumps when processing instructions other than <?xml ...?> have no pseudo-attributes (or any PI body lacking an =), e.g.:

$ { echo '<?xml version="1.0" encoding="UTF-8"?>'; echo '<?xml-stylesheet?>'; echo '<rss/>'; } | xq
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheetpanic: runtime error: index out of range [1] with length 1

Fixes #193.

Cause

In FormatXml, the xml.ProcInst branch split each pseudo-attribute on = and unconditionally read attrComponents[1]. An empty Inst (or one without =) yields a single-element slice, so the index panics.

Fix

Skip an empty PI body and only emit the =value part when the split actually produced two components.

Test plan

• Added test/data/xml/unformatted17.xml / formatted17.xml covering <?xml-stylesheet?> and <?xml-stylesheet type="text/xsl" href="style.xsl"?>, wired into TestFormatXml.
• go test ./... passes; the new case panics on master without this change.
• go vet ./... and gofmt clean.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.73%. Comparing base (7c06848) to head (d688a1d).

Files with missing lines	Patch %	Lines
internal/utils/utils.go	75.00%	1 Missing and 1 partial ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
- Coverage   80.90%   80.73%   -0.18%     
==========================================
  Files           5        5              
  Lines         707      711       +4     
==========================================
+ Hits          572      574       +2     
- Misses         93       94       +1     
- Partials       42       43       +1     

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