Update syntax error in bash script regarding LD_LIBRARY_PATH

See: apache/cloudberry#1624

Author: tuhaihe

docs/deployment/configure.md

@@ -30,7 +30,7 @@ The `configure` command sets up the build environment for Apache Cloudberry. Thi
 
 ```bash
 cd ~/cloudberry
-export LD_LIBRARY_PATH=/usr/local/cloudberry-db/lib:LD_LIBRARY_PATH
+export LD_LIBRARY_PATH=/usr/local/cloudberry-db/lib:${LD_LIBRARY_PATH:-""}
 ./configure --prefix=/usr/local/cloudberry-db \
             --disable-external-fts \
             --enable-gpcloud \

docs/deployment/quick-build.md

@@ -137,7 +137,7 @@ sudo chown -R gpadmin:gpadmin /usr/local/cloudberry-db
 
 # Run configure
 cd ~/cloudberry
-export LD_LIBRARY_PATH=/usr/local/cloudberry-db/lib:LD_LIBRARY_PATH
+export LD_LIBRARY_PATH=/usr/local/cloudberry-db/lib:${LD_LIBRARY_PATH:-""}
 ./configure --prefix=/usr/local/cloudberry-db \
             --disable-external-fts \
             --enable-gpcloud \

versioned_docs/version-2.x/deployment/configure.md

@@ -30,7 +30,7 @@ The `configure` command sets up the build environment for Apache Cloudberry. Thi
 
 ```bash
 cd ~/cloudberry
-export LD_LIBRARY_PATH=/usr/local/cloudberry-db/lib:LD_LIBRARY_PATH
+export LD_LIBRARY_PATH=/usr/local/cloudberry-db/lib:${LD_LIBRARY_PATH:-""}
 ./configure --prefix=/usr/local/cloudberry-db \
             --disable-external-fts \
             --enable-gpcloud \

versioned_docs/version-2.x/deployment/quick-build.md

@@ -145,7 +145,7 @@ sudo chown -R gpadmin:gpadmin /usr/local/cloudberry-db
 
 # Run configure
 cd ~/cloudberry
-export LD_LIBRARY_PATH=/usr/local/cloudberry-db/lib:LD_LIBRARY_PATH
+export LD_LIBRARY_PATH=/usr/local/cloudberry-db/lib:${LD_LIBRARY_PATH:-""}
 ./configure --prefix=/usr/local/cloudberry-db \
             --disable-external-fts \
             --enable-gpcloud \
@@ -326,7 +326,7 @@ sudo chown -R gpadmin:gpadmin /usr/local/cloudberry-db
 
 # Run configure
 cd ~/cloudberry
-export LD_LIBRARY_PATH=/usr/local/cloudberry-db/lib:LD_LIBRARY_PATH
+export LD_LIBRARY_PATH=/usr/local/cloudberry-db/lib:${LD_LIBRARY_PATH:-""}
 ./configure --prefix=/usr/local/cloudberry-db \
             --disable-external-fts \
             --enable-gpcloud \