refactor(api): replace length checks with isEmpty (#7214)

# Description of Changes

Stylistic problem reported by static analyzer. 


Changes:
* Replaced `sb.length() > 0` and `sb.length() == 0` with `!sb.isEmpty()`
and `sb.isEmpty()` for `StringBuilder`, `String`, and collections
throughout the codebase, improving readability and aligning with modern
Java best practices.


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## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [x] I have run `task check` to verify linters, typechecks, and tests
pass
- [x] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

app/common/src/main/java/stirling/software/common/pdf/PdfMarkdownConverter.java

@@ -983,7 +983,7 @@ public class PdfMarkdownConverter {
         ordered.sort(Comparator.comparingDouble((Line l) -> l.y).reversed());
         StringBuilder sb = new StringBuilder();
         for (Line l : ordered) {
-            if (sb.length() > 0) {
+            if (!sb.isEmpty()) {
                 sb.append(' ');
             }
             sb.append(l.text);

app/common/src/main/java/stirling/software/common/util/GeneralUtils.java

@@ -941,7 +941,7 @@ public class GeneralUtils {
             }
 
             // If no MAC address found, use hostname as fallback
-            if (sb.length() == 0) {
+            if (sb.isEmpty()) {
                 String hostname = InetAddress.getLocalHost().getHostName();
                 sb.append(hostname != null ? hostname : "unknown-host");
                 log.warn("No MAC address found, using hostname for fingerprint generation");

app/common/src/test/java/stirling/software/common/pdf/PdfMarkdownConverterTest.java

@@ -154,7 +154,7 @@ class PdfMarkdownConverterTest {
                     || isTableSeparatorRow(line)) {
                 continue;
             }
-            if (sb.length() > 0) {
+            if (!sb.isEmpty()) {
                 sb.append('\n');
             }
             sb.append(line);