Fix: importOrder() with single catch-all group strips blank lines added by greclipse(), causing non-idempotent formatting

[austek]

Summary

• importOrder() with no explicit groups (single catch-all) unconditionally replaced the entire import block, stripping any blank lines that a prior step like greclipse() had independently inserted between import groups.
• This created an infinite cycle: greclipse() adds blank lines → importOrder() strips them → greclipse() re-adds them → repeat (GitHub issue Maven Groovy importOrder prevents greclipse from checking and applying #899).
• Fix: when the sorted output contains no blank-line separators (single catch-all group) and the imports are already in the correct order, return the raw input unchanged, preserving any blank lines already present.

Test plan

• groovyImportsPreservesBlankLinesBetweenGroups — asserts that importOrder() with no args leaves an already-sorted Groovy import block with blank lines untouched.
• groovySortImportsStripsInterleavedBlankLines — asserts that sorting still works when imports are out of order; interleaved blank lines are removed as a side effect of the rewrite.

Fixes #899